Substrate for lithium ion battery separators and lithium ion battery separator

ABSTRACT

It is an object of the present invention to provide a substrate for lithium ion battery separators which has high adhesion to an inorganic particle layer, can be made thin and is excellent in tensile strength and cuttability and a lithium ion battery separator including the substrate for lithium ion battery separators. The substrate for lithium ion battery separators which contains heat-resistant fibers and synthetic resin short fibers contains fibrillated heat-resistant fibers having a modified freeness of not more than 300 ml as the heat-resistant fibers and has a content of the fibrillated heat-resistant fibers having a modified freeness of not more than 300 ml of not less than 1.0 mass % to less than 5.0 mass % based on the total of all the fiber components contained in the substrate. The modified freeness is a value measured in accordance with JIS P8121-2:2012 except that an 80-mesh wire net having a wire diameter of 0.14 mm and an opening of 0.18 mm is used as a screening plate and the concentration of a sample is 0.1%.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 16/323,334, filed Feb. 5, 2019, which is a national phase filing ofInternational Patent Application No. PCT/JP2017/031631, filed Sep. 1,2017, which claims the benefit of priority of Japanese PatentApplication No. 2016-175507, filed Sep. 8, 2016, Japanese PatentApplication No. 2016-219343, filed Nov. 10, 2016, Japanese PatentApplication No. 2016-219342, filed Nov. 10, 2016, Japanese PatentApplication No. 2016-223928, filed Nov. 17, 2016, Japanese PatentApplication No. 2017-127170, filed Jun. 29, 2017, and Japanese PatentApplication No. 2017-139776, filed Jul. 19, 2017, the entire contents ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a substrate for lithium ion batteryseparators and a lithium ion battery separator. The term “substrate forlithium ion battery separators” may be simply referred to as“substrate”. The term “lithium ion battery separator” may be simplyreferred to as “separator”.

BACKGROUND ART

The use of a lithium ion battery is now expanding to electricautomobiles in addition to portable equipment in the past. In the fieldof electric automobiles, as compared with portable equipment, alarge-sized lithium ion battery is employed.

As the lithium ion battery separator, a porous film made of a polyolefinsuch as polyethylene or polypropylene is often used. The porous filmmade of a polyolefin has a heat fuse function (shut-down function) thatit melts and closes pores to prevent the movement of a lithium ion andshut down a current when the inside temperature of a battery rises up to130° C. However, when the temperature further rises in some situation,the polyolefin has possibility that it itself melts to cause a shortcircuit, resulting in a thermal runaway. When a thermal runaway occursin a large-sized lithium ion battery, damage caused thereby becomesenormous. Then, a heat-resistant separator for preventing this thermalrunaway is attracting attention.

As the heat-resistant separator, there are disclosed a separatorincluding a polyester nonwoven fabric and a porous film made of anorganic polymer which swells with an electrolytic solution (refer toPatent Document 1) and a separator for electrochemical devices composedof a porous film containing at least one of a resin having a meltingpoint of 80 to 130° C. and a resin absorbing an electrolytic solution byheating to swell, a porous substrate which is a nonwoven fabric andfiller particles (inorganic particles) (refer to Patent Document 2).Each of these separators includes a substrate which is a nonwovenfabric, a porous film made of an organic polymer and a porous filmcontaining inorganic particles (inorganic particle layer), and theporous films are formed on the substrate by coating.

As the separator including a substrate which is a nonwoven fabric andporous films, there is proposed a separator whose heat resistance isfurther enhanced by a substrate containing heat-resistant fibers such asaromatic polyamide ((poly)aramid), aromatic polyester, polyamide-imideor polyimide fibers. For example, there is disclosed a substrate whichis a wet nonwoven fabric containing fibrillated heat-resistant fibersand synthetic short fibers as essential components, in which thepreferred mass ratio of the fibrillated heat-resistant fibers to thesynthetic short fibers is 7:1 to 1:19 (refer to Patent Document 3) or asubstrate which has polyaramid fibers, is bonded to fibers made of atleast one polymer and has a preferred mixing ratio of (the polyaramidfibers to the other polymer fibers) of 10:1 to 1:10 (refer to PatentDocument 4).

There are also disclosed a substrate which is a nonwoven fabriccontaining synthetic resin short fibers and fibrillated lyocell fibersas essential fibers and further containing fibrillated heat-resistantfibers in an amount of preferably 5 to 60 mass % (refer to PatentDocument 5) and a substrate which is a nonwoven fabric containing notless than 30 mass % of oriented crystallized polyester-based fibers andpreferably further containing fibrillated heat-resistant fibers in anamount of preferably 5 to 60 mass % based on the total of all the fibers(refer to Patent Document 6).

In the substrates disclosed by Patent Documents 3 to 6, the lower limitof the preferred content of the heat-resistant fibers is 5 mass %. Thatis, it is disclosed that a substrate having a heat-resistant fibercontent of not less than 5 mass % is preferred. However, since theheat-resistant fibers are mostly rigid, when the content thereof is notless than 5 mass %, the substrate is hardly smashed and it is difficultto reduce the thickness of the substrate. When the substrate is smashedby applying heat or a load, the resistance of the separator becomeshigh, when the substrate is cut to a predetermined width to be used in aseparator, it is difficult to cut the substrate in the slitting step,and it is also difficult to cut by welding in the step of laminating anelectrode. Further, as bonding force between the heat-resistant fibersand bonding force between the heat-resistant fiber and another fiber arelow, when the content of the heat-resistant fibers is not less than 5mass %, the tensile strength of the substrate becomes low and therefore,special care must be paid in the coating operation.

Patent Document 3 discloses a substrate which contains fibrillatedheat-resistant fibers as an essential component to solve problems suchas the deterioration of electrolytic solution retainability and a risein the internal resistance of the separator which are caused by closingpores in the substrate with inorganic particles filled in the nonwovenfabric. In Examples, a substrate having a content of the fibrillatedheat-resistant fibers of 20 to 60 mass % is manufactured. However, asubstrate having a high content of the fibrillated heat-resistant fibersmay have a problem that adhesion between the substrate and the inorganicparticle layer deteriorates since inorganic particles are accumulatedmainly on the surface of the substrate.

As the substrate on which the inorganic particle layer is to be formed,there is disclosed a substrate which contains polyethylene terephthalate(PET) fibers and achieves excellent coatability and post-processabilityby setting X-ray diffraction intensity derived from the PET fibers to aspecific range (refer to Patent Document 7). Patent Document 7 teachesthat fibers other than the PET fibers can be contained and the contentratio of the PET fibers and fibers other than the PET fibers ispreferably 100:0 to 70:30 in terms of mass. A substrate containing 20mass % of aramid fibers having a freeness of 300 ml is disclosed inExamples. However, Patent Document 7 merely teaches that “fibers otherthan the PET fibers can be contained as long as quality and productivityare not impaired” and does not take into consideration an effectobtained by blending the fibers other than the PET fibers.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A 2005-293891-   Patent Document 2: JP-A 2007-157723-   Patent Document 3: JP-A 2012-3873-   Patent Document 4: JP-A 2013-501082-   Patent Document 5: WO2011/046066-   Patent Document 6: JP-A 2012-134024-   Patent Document 7: JP-A 2013-179034

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

It is an object of the present invention to provide a substrate forlithium ion battery separators which has high adhesion to an inorganicparticle layer, can be made thin and is excellent in tensile strengthand cuttability and a lithium ion battery separator including thesubstrate for lithium ion battery separators.

Means for Solving the Problem

The inventors of the present invention conducted intensive studies tosolve the above problem and made the following invention.

-   -   (1-1) A substrate for lithium ion battery separators which        comprises heat-resistant fibers and synthetic resin short        fibers, wherein the substrate comprises fibrillated        heat-resistant fibers having a modified freeness of not more        than 300 ml as the heat-resistance fibers, and the content of        the fibrillated heat-resistant fibers having a modified freeness        of not more than 300 ml is not less than 1.0 mass % to less than        5.0 mass % based on the total of all the fiber components        contained in the substrate.    -   (1-2) The substrate for lithium ion battery separators in the        above paragraph (1-1), wherein the content of the synthetic        resin short fibers is 90.0 to 99.0 mass % based on the total of        all the fiber components contained in the substrate.    -   (1-3) The substrate for lithium ion battery separators in the        above paragraph (1-1), wherein the content of the synthetic        resin short fibers is more than 95.0 mass % to not more than        99.0 mass % based on the total of all the fiber components        contained in the substrate.

(2-1) A substrate for lithium ion battery separators which comprisesheat-resistant fibers and synthetic resin short fibers, wherein thesubstrate comprises fibrillated heat-resistant fibers having a modifiedfreeness of not more than 300 ml and fibrillated heat-resistant fibershaving a modified freeness of more than 300 ml as the heat-resistantfibers.

(2-2) The substrate for lithium ion battery separators in the aboveparagraph (2-1), wherein the total content of the fibrillatedheat-resistant fibers having a modified freeness of not more than 300 mland the fibrillated heat-resistance fibers having a modified freeness ofmore than 300 ml is not less than 2.0 mass % to less than 9.0 mass %based on the total of all the fiber components contained in thesubstrate.

(2-3) The substrate for lithium ion battery separators in the aboveparagraph (2-2), wherein the content of the fibrillated heat-resistantfibers having a modified freeness of not more than 300 ml is not lessthan 1.0 mass % to less than 5.0 mass % and the content of thefibrillated heat-resistant fibers having a modified freeness of morethan 300 ml is not less than 1.0 mass % to less than 4.0 mass % based onthe total of all the fiber components contained in the substrate.

(2-4) The substrate for lithium ion battery separators in the aboveparagraph (2-2) or (2-3), wherein the content of the synthetic resinshort fibers is 90.0 to 98.0 mass % based on the total of all the fibercomponents contained in the substrate.

(3-1) A substrate for lithium ion battery separators which comprisesheat-resistant fibers and synthetic resin short fibers, wherein thesubstrate comprises fibrillated heat-resistant fibers having a modifiedfreeness of not more than 300 ml and fibrid made of para-aromaticpolyamide having a modified freeness of not more than 300 ml as theheat-resistant fibers, and the content of the heat-resistant fibers isnot less than 1.0 mass % to less than 5.0 mass % based on the total ofall the fiber components contained in the substrate.

(3-2) The substrate for lithium ion battery separators in the aboveparagraph (3-1), wherein the content of the synthetic resin short fibersis 90.0 to 99.0 mass % based on the total of all the fiber componentscontained in the substrate.

(3-3) The substrate for lithium ion battery separators in the aboveparagraph (3-1), wherein the content of the synthetic resin short fibersis more than 95.0 mass % to not more than 99.0 mass % based on the totalof all the fiber components contained in the substrate.

(4-1) A substrate for lithium ion battery separators which comprisesheat-resistant fibers and synthetic resin short fibers, wherein thesubstrate contains fibrillated heat-resistant fibers having a modifiedfreeness of not more than 300 ml, fibrid made of para-aromatic polyamidehaving a modified freeness of not more than 300 ml, and fibrillatedheat-resistant fibers having a modified freeness of more than 300 ml asthe heat-resistant fibers, and the total content of the heat-resistantfibers is not less than 2.0 mass % to less than 9.0 mass % based on thetotal of all the fiber components contained in the substrate.

(4-2) The substrate for lithium ion battery separators in the aboveparagraph (4-1), wherein the content of the fibrillated heat-resistantfibers having a modified freeness of not more than 300 ml is not lessthan 0.6 mass % to less than 4.6 mass %, the content of the fibrid madeof para-aromatic polyamide having a modified freeness of not more than300 ml is not less than 0.4 mass % to less than 2.5 mass %, and thecontent of the fibrillated heat-resistant fibers having a modifiedfreeness of more than 300 ml is not less than 1.0 mass % to less than4.0 mass % based on the total of all the fiber components contained inthe substrate.

(4-3) The substrate for lithium ion battery separators in the aboveparagraph (4-1) or (4-2), wherein the content of the synthetic resinshort fibers is 90.0 to 98.0 mass % based on the total of all the fibercomponents contained in the substrate.

(5-1) A lithium ion battery separator which is a combination of at leastone composite selected from the group consisting of an inorganicparticle layer containing inorganic particles, organic particle layercontaining organic particles, porous film, microfiber layer, solidelectrolyte and gel-like electrolyte and the substrate for lithium ionbattery separators in any one of the above paragraphs (1-1) to (1-3),(2-1) to (2-4), (3-1) to (3-3) and (4-1) to (4-3).

(5-2) The lithium ion battery separator in the above paragraph (5-1),wherein the composite is an inorganic particle layer.

(5-3) The lithium ion battery separator in the above paragraph (5-1) or(5-2), wherein the inorganic particles are at least one type ofinorganic particles selected from the group consisting of alumina,alumina hydrate and magnesium compound.

(5-4) The lithium ion battery separator in the above paragraph (5-2)which is constituted such that an inorganic particle layer A containingmagnesium hydroxide having an average particle diameter of 2.0 to 4.0 μmand an inorganic particle layer B containing magnesium hydroxide havingan average particle diameter of not less than 0.5 μm to less than 2.0 μmare formed on one side of the substrate in the mentioned order.

(5-5) The lithium ion battery separator in the above paragraph (5-2)having an inorganic particle layer A containing magnesium hydroxidehaving an average particle diameter of 2.0 to 4.0 μm on one side of thesubstrate and an inorganic particle layer B containing magnesiumhydroxide having an average particle diameter of not less than 0.5 μm toless than 2.0 μm on the other side.

(5-6) The lithium ion battery separator in the above paragraph (5-4) or(5-5), wherein the coating weight of the inorganic particle layer A islower than the coating weight of the inorganic particle layer B.

The term “modified freeness” is “a value measured in accordance with JISP8121-2:2012 except that an 80-mesh wire net having a wire diameter of0.14 mm and an opening of 0.18 mm is used as a screening plate and theconcentration of a sample is 0.1%”.

Effect of the Invention

The substrate for lithium ion battery separators of the presentinvention can be made thin and is excellent in tensile strength andcuttability. The lithium ion battery separator of the present inventionwhich has the substrate for lithium ion battery separators of thepresent invention and an inorganic particle layer can attain an effectthat adhesion between the substrate and the inorganic particle layer ishigh.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the content ofheat-resistant fibers and the heat-resistant temperature of a substrate.

FIG. 2 is an optical photomicrograph showing the thermal fluidity of thesubstrate.

FIG. 3 is an optical photomicrograph showing the thermal fluidity of thesubstrate.

FIG. 4 is an electron photomicrograph of a substrate containingfibrillated heat-resistant fibers having a modified freeness of not morethan 300 ml as heat-resistant fibers.

FIG. 5 is an electron photomicrograph of a substrate containingfibrillated heat-resistant fibers having a modified freeness of not morethan 300 ml and fibrillated heat-resistant fibers having a modifiedfreeness of more than 300 ml as heat-resistant fibers.

FIG. 6 is an electron photomicrograph of a substrate containingfibrillated heat-resistant fibers having a modified freeness of not morethan 300 ml and fibrid made of para-aromatic polyamide having a modifiedfreeness of not more than 300 ml as heat-resistant fibers.

FIG. 7 is an electron photomicrograph of a substrate containingfibrillated heat-resistant fibers having a modified freeness of not morethan 300 ml, fibrillated heat-resistant fibers having a modifiedfreeness of more than 300 ml and fibrid made of para-aromatic polyamidehaving a modified freeness of not more than 300 ml as heat-resistantfibers.

FIG. 8 is an electron photomicrograph of a substrate containing noheat-resistant fibers.

PRACTICAL EMBODIMENT OF THE INVENTION

In the present invention, the substrate for lithium ion batteryseparators is a precursor sheet of a lithium ion battery separator whichbecomes a lithium ion battery separator by combining at least onecomposite selected from an inorganic particle layer containing inorganicparticles, an organic particle layer containing organic particles, aporous film, a nanofiber layer, a solid electrolyte and a gel-likeelectrolyte. That is, the substrate of the present invention does notbecome a lithium ion battery separator by itself. From the viewpoint ofheat resistance, the composite is preferably an inorganic particlelayer. That is, a separator having a substrate and an inorganic particlelayer is preferred.

In the present invention, as the inorganic particles, alumina's such asα-alumina, β-alumina and γ-alumina; alumina hydrates such as boehmite;and magnesium compounds such as magnesium oxide and magnesium hydroxidemay be used. Out of these, α-alumina, alumina hydrates and magnesiumhydroxide are preferably used as they have high stability to anelectrolyte used in a lithium ion battery.

As the organic particles, polyethylene, polypropylene,polyacrylonitrile, polymethyl methacrylate, polyethylene oxide,polystyrene, polyvinylidene fluoride, ethylene-vinyl monomer copolymerand polyolefin wax are used.

As a resin for the porous film, any resin is acceptable if it can beused to form a film but polyolefin-based resins such aspolyethylene-based resin and polypropylene-based resin are preferred. Asthe polyethylene-based resin, not only a polyethylene-based resin alonesuch as super low-density polyethylene, low-density polyethylene, linearlow-density polyethylene, intermediate-density polyethylene,high-density polyethylene or super high-density polyethylene but also anethylene-propylene copolymer and a mixture of a polyethylene-based resinand another polyolefin-based resin are used. Examples of thepolypropylene-based resin include homopropylene (propylene homopolymer),and random copolymers or block copolymers of an α-olefin such asethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-noneneor 1-decene and propylene. As means of combining the substrate and theporous film, lamination integration by thermal lamination and laminationintegration using an adhesive may be employed.

The lithium ion battery in the present invention refers to a secondarybattery in which a lithium ion moves between an anode and a cathode upondischarge and charge. Examples of the lithium ion battery include alithium ion secondary battery and a lithium ion polymer secondarybattery. The lithium ion battery includes a lithium ion secondarybattery comprising a lithium storage material as a negative electrodeactive material and a metal lithium secondary battery comprising metallithium as a negative electrode active material.

The lithium ion battery includes a cathode, a separator and an anode asmembers. In general, it has a structure that a cathode, a separator andan anode are laminated in the mentioned order. An electrolytic solutionis absorbed into the cathode, the anode and the separator. The types ofthe laminate structure include a cylindrical type in which all themembers are laminated together and then rolled, a roll flat type inwhich the cylindrical type is squashed to form two flat surfaces andcurved both end parts, a ninety-nine-fold type in which leaf electrodesare inserted into a separator which has been folded 99 times and a leaflaminate type in which a leaf separator and leaf electrodes arelaminated together.

A lithium storage material is used as the negative electrode activematerial of the lithium ion battery. Examples of the lithium storagematerial include carbon-based materials, silicon-based materials andcomposite oxides of a transition metal and lithium. The carbon-basedmaterials are preferably used as they are excellent in balance betweenthe amount of lithium able to be stored per mass and the difficulty ofdeterioration caused by the absorption and release of lithium. Thecarbon-based materials include graphite's such as natural graphite andartificial graphite; amorphous carbons such as hard carbon, soft carbonand mesoporous carbon; and nanocarbon materials such as carbon nanotubeand graphene. The silicon-based materials are preferably used as theamount of lithium able to be stored per mass is large. The silicon-basedmaterials include silicon, silicon monoxide (SiO) and silicon dioxide(SiO₂). Lithium titanate which is a type of a composite oxide of atransition metal and lithium is preferably used as deterioration causedby the absorption and release of lithium hardly occurs.

An example of the anode of the lithium ion battery is an electrodeproduced by coating an anode material containing the above negativeelectrode active material on metal foil. The anode material may be mixedwith a binder such as polyvinylidene fluoride or styrene-butadienecopolymer; conductive agent such as carbon black or nanocarbon material;dispersant; and thickener as required. As the metal used in the metalfoil, copper or aluminum is used.

Examples of the positive electrode active material of the lithium ionbattery include a composite oxide of a transition metal and lithium, acomposite salt having an olivine structure of a transition metal andlithium, and sulfur. An example of the composite oxide of a transitionmetal and lithium is a composite oxide of at least one transition metalselected from cobalt, nickel and manganese and lithium. A typical metalsuch as aluminum or magnesium and a transition metal such as titanium orchromium may be further combined with the composite oxide. An example ofthe composite salt having an olivine structure of a transition metal andlithium is a composite salt having an olivine structure of at least onetransition metal selected from iron and manganese and lithium.

An example of the cathode of the lithium ion battery is an electrodeproduced by coating a cathode material containing the above positiveelectrode active material on metal foil. The cathode material may bemixed with a binder such as polyvinylidene fluoride or acrylic acidester copolymer; conductive agent such as carbon black or nanocarbonmaterial; dispersant; and thickener as required. An example of the metalused in the metal foil is aluminum.

Examples of the electrolytic solution of the lithium ion battery includea solution prepared by dissolving a lithium salt in a polar solvent anda solution prepared by dissolving a lithium salt in an ion liquid.Examples of the polar solvent used in the electrolytic solution of thelithium secondary battery include carbonates such as propylene carbonate(PC), ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methylcarbonate (EMC); and fatty acid esters such as ethyl acetate, propylacetate and ethyl propionate. Examples of the lithium salt used in theelectrolytic solution of the lithium secondary battery include lithiumhexafluorophosphate (LiPF₆) and lithium tetrafluoroborate (LiBF₄). Asthe solid electrolyte, a solution prepared by dissolving a lithium saltin polyethylene glycol or derivative thereof, polymethacrylic acidderivative, polysiloxane or derivative thereof, or gel-like polymer suchas polyvinylidene fluoride is used.

The substrate of the present invention contains heat-resistant fibersand synthetic resin short fibers, and fibrillated heat-resistant fibershaving a modified freeness of not more than 300 ml are contained as theheat-resistant fibers as an essential component.

In the present invention, the following expressions may be used.

Fibrillated heat-resistant fibers having a modified freeness of not morethan 300 ml: fibrillated heat-resistant fibers (I)

Fibrillated heat-resistant fibers having a modified freeness of morethan 300 ml: fibrillated heat-resistant fibers (II) Fibrid made ofpara-aromatic polyamide having a modified freeness of not more than 300ml: fibrid

The reason that the substrate of the present invention contains thefibrillated heat-resistant fibers (I) as an essential component is givenbelow. FIG. 1 is a graph showing the relationship between the content ofthe heat-resistant fibers and the heat-resistant temperature of thesubstrate. Polyethylene terephthalate (PET)-based synthetic resin shortfibers are used as the synthetic resin short fibers. A substrate (12 cmin a width direction×25 cm in a flow direction) was prepared. Then, thefour corners of the substrate were fixed by Cellophane tape withoutapplying tension to an aluminum frame measuring 20 cm×20 cm.Subsequently, a hot air gun (trade name: WEP 858D mini-station) wasfixed to ensure that the distance between the ejection port having adiameter of 7.5 mm of the hot air gun and the central point of thesubstrate became 3 cm. Then, hot air was applied to the substrate at aflow rate of 5.3 l/min for 20 seconds to take the maximum temperature (°C.) at which a hole was not formed as “heat-resistant temperature”. Whenthe content of the heat-resistant fibers is 3 mass %, the heat-resistanttemperature of the substrate containing the fibrillated heat-resistantfibers (I) is 278° C. whereas the heat-resistant temperature of thesubstrate containing the fibrillated heat-resistant fibers (II) is 270°C. When the content of the heat-resistant fibers is 3.5 mass %, theheat-resistant temperature of the substrate containing the fibrillatedheat-resistant fibers (I) is 280° C. whereas the heat-resistanttemperature of the substrate containing fibrid is 275° C. Further, asthe content of the heat-resistant fibers becomes higher, theheat-resistant temperature of the substrate containing the fibrillatedheat-resistant fibers (I) tends to become higher but the heat-resistanttemperature of the substrate containing the fibrillated heat-resistantfibers (II) tends to become lower.

FIG. 2 and FIG. 3 are optical photomicrographs showing the thermalfluidity of a substrate when the heat-resistant fiber content waschanged. Substrates (1 cm×1 cm) were placed one upon another to ensurethat the total weight became 0.03 g and sandwiched between two slideglass plates and fixed by aluminum tape. Then, three iron blocks (40 geach) were placed on the assembly which was then put into an electricfurnace for 20 minutes. The areas of the substrates taken out from theelectric furnace were measured by an optical microscope. FIG. 2 arephotos taken when the temperature of the electric furnace was 300° C.,wherein FIG. 2(A) shows a substrate having a content of theheat-resistant fibers of 0 mass % and FIG. 2(C) shows a substrate havinga content of the heat-resistant fibers (II) of 3.0 mass %. When thetemperature of the electric furnace is 300° C., it is understood thateven the substrate containing no heat-resistant fibers has low thermalfluidity. FIG. 3 are photos taken when the temperature of the electricfurnace was 350° C., wherein FIG. 3(A) shows a substrate having acontent of the heat-resistant fibers of 0 mass %, FIG. 3(B) shows asubstrate having a content of the fibrillated heat-resistant fibers (I)of 3.5 mass %, FIG. 3(C) shows a substrate having a content of thefibrillated heat-resistant fibers (II) of 3.0 mass %, and FIG. 3(D)shows a substrate having a content of fibrid of 3.5 mass %. Thesubstrates taken out from the electric furnace flowed by heat and had anarea (cm²) of (A) 4.9, (B) 0.7, (C) 1.7 and (D) 0.7. It is understoodthat a heat flow is suppressed in the substrate containing thefibrillated heat-resistant fibers (I) or fibrid.

It is understood from the evaluation results of heat-resistanttemperature and thermal fluidity that the substrate containing thefibrillated heat-resistant fibers (I) as an essential component isexcellent as it has the highest heat-resistant temperature andsuppresses a heat flow.

The substrate (1) for lithium ion battery separators of the presentinvention contains the fibrillated heat-resistant fibers (I) asheat-resistant fibers, and the content of the fibrillated heat-resistantfibers (I) is not less than 1.0 mass % to less than 5.0 mass % based onthe total of all the fiber components contained in the substrate.

The content of the fibrillated heat-resistant fibers (I) is not lessthan 1.0 mass % to less than 5.0 mass %, preferably not less than 2.0mass %, more preferably not less than 3.0 mass % based on the total ofall the fiber components contained in the substrate (1) of the presentinvention. The content is preferably is less than 4.5 mass %. When thecontent of the fibrillated heat-resistant fibers (I) is not less than5.0 mass %, it is difficult to smash the substrate to reduce thethickness of the substrate. Further, tensile strength lowers and acoating solution for forming an inorganic particle layer hardlypermeates the substrate, whereby the adhesion of the inorganic particlelayer deteriorates and internal resistance becomes worse. Meanwhile,when the content of the fibrillated heat-resistant fibers (I) is lessthan 1.0 mass %, though the tensile strength of the substrate becomeshigh, the effect of improving a leakage current becomes low. When theweight of the substrate is reduced, the coating solution easilypermeates and therefore goes through to the rear side to stain a runningroll and degrade the surface quality of the separator.

Patent Document 3 (JP-A 2012-3873), Patent Document (WO2011/046066),Patent Document 6 (JP-A 2012-134024) and Patent Document 7 (JP-A2013-179034) listed in the Prior Art Documents disclose substratescontaining fibrillated heat-resistant fibers. The lowest value of thepreferred content of the fibrillated heat-resistant fibers is 5 mass %.In Examples of these patent documents, mainly substrates having athickness of not less than 17 μm were manufactured and evaluated. Whenthe inventors of the present invention studied these substrates indetail, they found the following fact. For example, in the evaluation ofthe internal resistance of the battery, even when a substrate having athickness of not less than 17 μm had a content of the fibrillatedheat-resistant fibers of 10.0 mass % or 4.0 mass %, it showed lowinternal resistance. When a substrate having a small thickness of notmore than 15 μm had a content of the fibrillated heat-resistant fibers(I) of 4.0 mass %, it showed low internal resistance. However, when thesubstrate had a content of the fibrillated heat-resistance fibers (I) of10.0 mass %, it showed high internal resistance. Even when the substrate(1) of the present invention having a content of the fibrillatedheat-resistant fibers (I) of not less than 1.0 mass % to less than 5.0mass % is thick or thin, it has an effect that cuttability and adhesionto the inorganic particle layer are excellent and an advantageous effectthat internal resistance can be reduced.

The content of the synthetic resin short fibers is preferably not lessthan 90.0 mass %, more preferably not less than 92.0 mass %, much morepreferably not less than 94.0 mass %, particularly preferably more than95.0 mass % based on the total of all the fiber components contained inthe substrate (1) of the present invention. The content is preferablynot more than 99.0 mass %, more preferably not more than 98.0 mass %,much more preferably not more than 96.0 mass %. When the content of thesynthetic resin short fibers is more than 99.0 mass %, though thetensile strength of the substrate becomes high, the effect of improvinga leakage current may become low. When the content of the syntheticresin short fibers is less than 90.0 mass % and the weight of thesubstrate is reduced, mechanical strength becomes low, whereby thesubstrate may be broken at the time of forming the inorganic particlelayer.

The most preferred substrate (1) of the present invention is a substratecontaining the fibrillated heat-resistant fibers (I) and the syntheticresin short fibers as fiber components. In this case, the content of thesynthetic resin short fibers is more than 95.0 mass % to not more than99.0 mass % based on the total of all the fiber components contained inthe substrate (1) of the present invention. It is preferably not morethan 98.0 mass %, more preferably not more than 97.0 mass %. It ispreferably more than 95.5 mass %.

The substrate (2) for lithium ion battery separators of the presentinvention contains the fibrillated heat-resistant fibers (I) and thefibrillated heat-resistant fibers (II) as heat-resistant fibers.

The total content of the fibrillated heat-resistant fibers (I) and thefibrillated heat-resistant fibers (II) is preferably not less than 2.0mass % to less than 9.0 mass % based on the total of all the fibercomponents contained in the substrate (2) of the present invention. Itis more preferably not less than 3.0 mass %, much more preferably notless than 3.5 mass %. It is more preferably less than 8.0 mass %, muchmore preferably less than 6.0 mass %, particularly preferably less than5.0 mass %. When the total content of the fibrillated heat-resistantfibers (I) and (II) is not less than 9.0 mass %, it is difficult toreduce the thickness of the substrate. Further, since tensile strengthlowers and the coating solution for forming the inorganic particle layerhardly permeates the substrate, whereby adhesion to the inorganicparticle layer may deteriorate and internal resistance may become worse.When the total content of the fibrillated heat-resistant fibers (I) and(II) is less than 2.0 mass %, though the tensile strength of thesubstrate becomes high, the effect of improving a leakage current maybecome low. When the weight of the substrate is reduced, the coatingsolution easily permeates and goes through to the rear side to stain atraveling roll and degrade the surface quality of the separator.

It is preferred that the content of the fibrillated heat-resistantfibers (I) should be not less than 1.0 mass % to less than 5.0 mass %and that the content of the fibrillated heat-resistant fibers (II)should be not less than 1.0 mass % to less than 4.0 mass % based on thetotal of all the fiber components contained in the substrate (2) of thepresent invention.

The content of the fibrillated heat-resistant fibers (I) is preferablynot less than 1.0 mass % to less than 5.0 mass %, more preferably notless than 2.0 mass % to less than 5.0 mass %, much more preferably notless than 2.5 mass % to less than 4.0 mass %, particularly preferablynot less than 2.5 mass % to less than 3.5 mass % based on the total ofall the fiber components contained in the substrate (2) of the presentinvention. Within this range, the fibrillated heat-resistant fibers (I)include a small number of thick stem fibers which are not fibrillatedand a large number of fibrillated fine fibers, whereby there is noobstacle to the control of the thickness of the substrate and thecuttability of the separator, and the diameter of each pore becomessmall. Therefore, the coating solution hardly goes through to the rearside and the retainability of the electrolytic solution becomes high,thereby producing the effect of improving the resistance of theseparator. When the content is not less than 5.0 mass %, tensilestrength may lower, and the inorganic particles are accumulated mainlyon the surface of the substrate, whereby the resistance of the separatormay become worse and adhesion between the substrate and the inorganicparticle layer may deteriorate. When the content is less than 1.0 mass%, the effect obtained by blending the fibrillated heat-resistant fibers(I) may be rarely observed.

The content of the fibrillated heat-resistant fibers (II) is preferablynot less than 1.0 mass % to less than 4.0 mass %, more preferably 1.0 to3.0 mass %, much more preferably 1.0 to 2.0 mass %, particularlypreferably 1.0 to 1.5 mass % based on the total of all the fibercomponents contained in the substrate (2) of the present invention. Whenthe fibrillated heat-resistant fibers (II) are used in this range,though the fibrillated heat-resistant fibers (I) are contained, there isno obstacle to the control of the thickness of the substrate and thecuttability of the separator, the resistance of the separator rarelybecomes worse, and adhesion between the substrate and the inorganicparticle layer rarely deteriorates. Since the fibrillatedheat-resistance fibers (II) include relatively thick stem fibersremaining besides the fibrillated fine fibers, the compressionresistance of the substrate improves and the short-circuit resistance ofthe separator becomes high.

The content of the synthetic resin short fibers is preferably not lessthan 90.0 mass %, more preferably not less than 92 mass %, much morepreferably not less than 94.0 mass %, particularly preferably more than95.0 mass % based on the total of all the fiber components contained inthe substrate (2) of the present invention. The content is preferablynot more than 98.0 mass %, more preferably not more than 97.0 mass %,much more preferably not more than 96.0 mass %. When the content of thesynthetic resin short fibers is more than 98.0 mass %, though thetensile strength of the substrate becomes high, the effect of improvinga leakage current may become low in the case of the low weight of thesubstrate. When the content of the synthetic resin short fibers is lessthan 90.0 mass % and the weight of the substrate is reduced, mechanicalstrength becomes low, whereby the substrate may be broken at the time offorming the inorganic particle layer.

The most preferred substrate (2) of the present invention is a substratecontaining the fibrillated heat-resistant fibers (I) and (II) and thesynthetic resin short fibers as fiber components. In this case, thecontent of the synthetic resin short fibers is preferably more than 91.0mass % to not more than 98.0 mass % based on the total of all the fibercomponents contained in the substrate of the present invention. It ismore preferably not more than 97.0 mass %, much more preferably not morethan 96.0 mass %. It is more preferably more than 95.0 mass %.

The substrate (3) for lithium ion battery separators of the presentinvention contains the fibrillated heat-resistant fibers (I) and fibridas heat-resistant fibers.

The total content of the fibrillated heat-resistance fibers (I) andfibrid is not less than 1.0 mass % to less than 5.0 mass % based on thetotal of all the fiber components contained in the substrate (3) of thepresent invention. It is preferably not less than 1.6 mass %, morepreferably not less than 2.1 mass %, particularly preferably not lessthan 2.5 mass %. When the total content of the fibrillatedheat-resistant fibers (I) and fibrid is not less than 5.0 mass %, it isdifficult to reduce the thickness of the substrate. Since the inorganicparticle layer for forming the inorganic particle layer hardly permeatesthe substrate, adhesion to the inorganic particle layer may deteriorateand internal resistance may become worse. When the total content of thefibrillated heat-resistant fibers (I) and fibrid is less than 1.0 mass%, though the tensile strength of the substrate becomes high, the effectof improving a leakage current may become low. When the weight of thesubstrate is reduced, the coating solution easily permeates, whereby theinorganic particle layer may go through to the rear side to stain thetraveling roll of a coating device and degrade the surface quality ofthe separator.

In the substrate (3) of the present invention, the content of thefibrillated heat-resistant fibers (I) is preferably more than 50 mass %,more preferably not less than 60 mass %, much more preferably not lessthan 70 mass %, particularly preferably not less than 80 mass % based onthe total content of the fibrillated heat-resistant fibers (I) andfibrid. The content of the fibrillated heat-resistant fibers (I) ispreferably not more than 92 mass %, more preferably not more than 88mass %, much more preferably not more than 84 mass %.

In the substrate (3) of the present invention, when the content of thefibrillated heat-resistant fibers (I) is more than 50 mass % and thefibrillated heat-resistant fibers (I) are used in combination withfibrid, the substrate can be made thin, there is no obstacle to thecuttability of the separator, and the diameters of pores become small,whereby the coating solution hardly goes through to the rear side andthe retainability of the electrolytic solution improves with the resultthat the resistance of the separator hardly becomes worse and adhesionto the inorganic particle layer hardly deteriorates. The fibrid has athin leaf-like fibrous form and greatly shrinks when moisture existentin the crystal structure is dried off and removed, thereby strengtheninga network formed by the fibrillated heat-resistant fibers (I) and thesynthetic resin short fibers. Therefore, even when the substrate has lowweight, the strength characteristic of the substrate can be maintainedand a leakage current can be prevented, thereby improving theshort-circuit resistance of the separator.

In the substrate (3) of the present invention, when the content of thefibrillated heat-resistant fibers (I) is not more than 50 mass % and thesubstrate is thin, internal resistance may rise. When the content of thefibrillated heat-resistant fibers (I) is not more than 50 mass % and theweight of the substrate is reduced, the strength characteristic of thesubstrate may be hardly maintained and a leakage current may be hardlyprevented. When the content of the fibrillated heat-resistant fibers (I)is more than 92 mass %, tensile strength may lower, and the inorganicparticle layer hardly permeates the substrate, whereby the inorganicparticles are accumulated mainly on the surface of the substrate withthe result that the resistance of the separator may become worse andadhesion between the substrate and the inorganic particle layer maydeteriorate.

The content of the synthetic resin short fibers is preferably not lessthan 90.0 mass %, more preferably not less than 92.0 mass %, much morepreferably not less than 94.0 mass %, particularly preferably more than95.0 mass % based on the total of all the fiber components contained inthe substrate (3) of the present invention. It is preferably not morethan 99.0 mass %, more preferably not more than 98.0 mass %, much morepreferably not more than 97.0 mass %. When the content of the syntheticresin short fibers is more than 99.0 mass %, though the tensile strengthof the substrate becomes high, the effect of improving a leakage currentmay become low. When the content of the synthetic resin short fibers isless than 90.0 mass % and the weight of the substrate is reduced,mechanical strength becomes low, whereby the substrate may be broken atthe time of forming the inorganic particle layer.

The most preferred substrate (3) of the present invention is a substratecontaining the fibrillated heat-resistant fibers (I), fibrid and thesynthetic resin short fibers as fiber components. In this case, thecontent of the synthetic resin short fibers is preferably 96.0 to 99.0mass % based on the total of all the fiber components contained in thesubstrate (3) of the present invention. It is more preferably not morethan 98.0 mass %, much more preferably not more than 97.0 mass %. It isparticularly preferably not less than 96.5 mass %.

The substrate (4) for lithium ion battery separators of the presentinvention contains the fibrillated heat-resistant fibers (I), thefibrillated heat-resistant fibers (II) and fibrid as heat-resistantfibers.

The total content of the fibrillated heat-resistant fibers (I) and (II)and fibrid is preferably not less than 2.0 mass % to less than 9.0 mass%, more preferably not less than 3.0 mass % to less than 9.0 mass %,much more preferably not less than 3.5 mass % to less than 7.0 mass %,particularly preferably not less than 3.5 mass % to less than 5.0 mass %based on the total of all the fiber components contained in thesubstrate (4) of the present invention. When the total content of thetwo different fibrillated heat-resistant fibers (I) and (II) and fibridis not less than 9.0 mass %, it is difficult to reduce the thickness ofthe substrate. Tensile strength may lower, and the coating solution forforming the inorganic particle layer may hardly permeate the substrate,whereby adhesion to the inorganic particle layer may deteriorate andinternal resistance may become worse. When the total content of the wodifferent fibrillated heat-resistant fibers (I) and (II) and fibrid isless than 2.0 mass %, though the tensile strength of the substratebecomes high, the effect of improving a leakage current may become low.When the weight of the substrate is reduced, the coating solution easilypermeates and may go through to the rear side to stain a travelling rolland degrade the surface quality of the separator.

The content of the fibrillated heat-resistant fibers (I) is preferablynot less than 0.6 mass % to less than 4.6 mass %, more preferably notless than 1.0 mass % to less than 4.6 mass %, much more preferably notless than 1.5 mass % to less than 4.0 mass %, particularly preferably2.0 to 3.0 mass % based on the total of all the fiber componentscontained in the substrate (4) of the present invention. When thefibrillated heat-resistant fibers (I) are used in this range, thefibrillated heat-resistant fibers (I) include a small number of thickstem fibers which are not fibrillated and a large number of fibrillatedfine fibers, whereby there is no obstacle to the control of thethickness of the substrate and the cuttability of the separator, and thediameters of pores become small. Therefore, the coating solution hardlygoes through to the rear side and the retainability of the electrolyticsolution improves with the result that the resistance of the separatorbecomes better. When the content of the fibrillated heat-resistantfibers (I) is not less than 4.6 mass %, tensile strength may lower, andthe resistance of the separator may become worse and adhesion betweenthe substrate and the inorganic particle layer may deteriorate since theinorganic particles are accumulated mainly on the surface of thesubstrate. When the content of the fibrillated heat-resistant fibers (I)is less than 0.6 mass %, the effect obtained by blending the fibrillatedheat-resistant fibers may not be observed.

The content of the fibrillated heat-resistant fibers (II) is preferablynot less than 1.0 mass % to less than 4.0 mass %, more preferably 1.0 to3.0 mass %, much more preferably 1.0 to 2.0 mass %, particularlypreferably 1.0 to 1.5 mass % based on the total of all the fibercomponents contained in the substrate (4) of the present invention. Whenthe fibrillated heat-resistant fibers (II) are used in this range,though fibrid and the fibrillated heat-resistant fibers (I) arecontained, there is no obstacle to the control of the thickness of thesubstrate and the cuttability of the separator, the resistance of theseparator rarely becomes worse and adhesion between the substrate andthe inorganic particle layer rarely deteriorates. Since the fibrillatedheat-resistant fibers (II) include relatively thick stem fibersremaining besides the fibrillated fine fibers, the compressionresistance of the substrate and the short-circuit resistance of theseparator improve.

The content of fibrid is preferably not less than 0.4 mass % to lessthan 2.5 mass %, more preferably not less than 0.4 mass % to less than2.0 mass %, much more preferably not less than 0.4 mass % to less than1.5 mass %, particularly preferably not less than 0.4 mass % to lessthan 1.0 mass % based on the total of all the fiber components containedin the substrate (4) of the present invention. When fibrid is used inthis range, though the fibrillated heat-resistant fibers (I) and (II)are contained, there is no obstacle to the control of the thickness ofthe substrate and the cuttability of the separator, the resistance ofthe separator rarely becomes worse, and adhesion between the substrateand the inorganic particle layer rarely deteriorates. Further, thefibrid has a thin leaf-like form and greatly shrinks when moistureexistent in the crystal structure is dried off and removed, therebystrengthening a network formed by the fibrillated heat-resistant fibers(I) and (II) and the synthetic resin short fibers. Therefore, even whenthe weight is low, the strength characteristic of the substrate can bemaintained and a leakage current can be prevented, thereby improving theshort-circuit resistance of the separator.

Although the total content of the fibrillated heat-resistant fibers (I)and (II) and fibrid is not less than 2.0 mass % to less than 9.0 mass %based on the total of all the fiber components contained in thesubstrate (4) of the present invention, when the total content of thefibrillated heat-resistant fibers (I) and fibrid is not less than 5.0mass %, the inorganic particle layer hardly permeates the substrate,whereby adhesion to the inorganic particle layer may deteriorate. Inaddition, when the content of fibrid is not less than 2.5 mass % and theweight of the substrate is low, internal resistance may become worse.

The content of the synthetic resin short fibers is preferably not lessthan 90.0 mass %, more preferably not less than 92.0 mass %, much morepreferably not less than 93.0 mass %, particularly preferably more than95.0 mass % based on the total of all the fiber components contained inthe substrate (4) of the present invention. It is preferably not morethan 98.0 mass %, more preferably not more than 97.5 mass %, much morepreferably not more than 97.0 mass %, particularly preferably not morethan 96.5 mass %. When the content of the synthetic resin short fibersis more than 98.0 mass %, though the tensile strength of the substratebecomes high, the effect of improving a leakage current may become low.When the content of the synthetic resin short fiber is less than 90.0mass % and the weight of the substrate is reduced, mechanical strengthbecomes low, whereby the substrate may be broken at the time of formingthe inorganic particle layer.

In the present invention, as the fibrillated heat-resistant fibers,heat-resistant fibers which are made of a heat-resistant resin such aswholly aromatic polyamide (aramid), wholly aromatic polyester,polyimide, polyamide imide, polyether ether ketone, polyphenylenesulfide, polybenzimidazole, poly-p-phenylene benzobisthiazole,poly-p-phenylene benzobisoxazole or polytetrafluoroethylene andfibrillated are used. Out of these, wholly aromatic polyamide which hashigh affinity for an electrolytic solution and is easily fibrillated ispreferred.

The fibrillated heat-resistant fibers can be obtained by processingheat-resistant fibers with a refiner, beater, mill, grinding machine,rotary homogenizer which applies shear force with a high-speed rotaryblade, double cylinder type high-speed homogenizer which produces shearforce between an cylindrical inner blade rotating at a high speed and afixed outer blade, ultrasonic crusher which refines with ultrasonicshock, or high-pressure homogenizer which lets a fiber suspension passthrough a small-diameter orifice by applying a pressure difference toaccelerate the speed of the suspension and collides the fiber suspensionto rapidly decelerate its speed so as to apply shear force and cuttingforce to the fibers.

In the present invention, the term “fibrid” refers to a thin leaf-likeor flaky piece having fine fibrils and fine heat-resistant fibers inwhich a water molecule or moisture is existent in a crystal structure inan amorphous state without forming a strong fiber crystal structure. Thefibrid is obtained by collecting a product obtained by introducing afiber forming high-molecular weight polymer solution into an aqueouscoagulation bath without drying it and fibrillating it by beating asrequired. For example, it is fibrid which is produced by mixing apolymer solution with a precipitating agent for it in the presence ofshear force, or an amorphous water-containing product having molecularorientation formed from a high-molecular weight polymer solution showingoptical anisotropy and may be beaten as required.

The fibrid can be obtained by beating with a refiner, beater, mill,grinding machine, rotary homogenizer which applies shear force with ahigh-speed rotary blade, double cylinder type high-speed homogenizerwhich produces shear force between an cylindrical inner blade rotatingat a high speed and a fixed outer blade, ultrasonic crusher whichrefines with ultrasonic shock, or high-pressure homogenizer which lets afiber suspension pass through a small-diameter orifice by applying apressure difference to accelerate the speed of the suspension andcollides the fiber suspension to rapidly decelerate its speed so as toapply shear force and cutting force to the fibers.

In the present invention, the fibrid greatly shrinks when moistureexistent in the crystal structure is removed by heating or pressurereduction to strengthen the fiber network, thereby improving thestrength characteristic of the substrate.

The modified freeness of the fibrillated heat-resistant fibers (I) inthe present invention is not more than 300 ml, preferably 0 to 200 ml,more preferably 0 to 100 ml. When the modified freeness of all thefibrillated heat-resistant fibers is more than 300 ml, a large number ofthick stem fibers are existent, thereby making it difficult to controlthe thickness and deteriorating the cuttability of the separator. Sincethe permeability of an ion is hindered and the retainability of theelectrolytic solution is deteriorated by the existence of thick stemfibers, the resistance of the separator becomes high. Further, there isa great need to increase the load or temperature of a heat calender forthe control of thickness, whereby the fusion area of a binder fiber iswidened and the permeability of an ion is hindered, thereby furtherraising the resistance of the separator. When the modified freeness isless than 0 ml, the fibrillation of the fibrillated heat-resistancefibers proceeds too much, whereby the number of fine fibers to be bondedby a predetermined amount of a binder fiber is increased and therefore,tensile strength may lower.

The modified freeness of the fibrillated heat-resistant fibers (II) inthe present invention is more than 300 ml, preferably more than 300 mlto less than 700 ml, more preferably more than 300 ml to less than 600ml, much more preferably more than 300 ml to less than 450 ml. When themodified freeness is not less than 700 ml, fibrillation does not proceedmuch, whereby it is difficult to control the thickness and thecuttability of the separator may deteriorate due to the existence of alarge number of thick stem fibers. Since the permeability of an ion ishindered and the retainability of the electrolytic solution isdeteriorated by the existence of thick stem fibers, the resistance ofthe separator may become high.

In the present invention, the modified freeness of the fibrid ispreferably 0 to 300 ml, more preferably 0 to 200 ml, much morepreferably 0 to 100 ml. When the modified freeness is more than 300 ml,the fiber width of the fibrid becomes large and the resistance of theseparator may become high.

The texture (density unevenness) of the substrate becomes worse and themechanical strength of the substrate becomes low, whereby the substratemay be broken at the time of forming the inorganic particle layer.Further, the cuttability of the separator may deteriorate.

Preferably, the fibrillated heat-resistance fibers (I) have amass-weighted average fiber length of 0.02 to 1.00 mm. Thelength-weighted average fiber length is preferably 0.02 to 0.50 mm. Whenthe average fiber length is smaller than the preferred range, thefibrillated heat-resistant fibers may drop off from the substrate. Whenthe average fiber length is larger than the preferred range, thedefibration of fibers deteriorates and a dispersion failure tends tooccur.

Preferably, the fibrillated heat-resistant fibers (II) have amass-weighted average fiber length of 1.00 to 1.50 mm. Thelength-weighted average fiber length is preferably 0.50 to 1.00 mm. Whenthe average fiber length is smaller than the above range, the effect ofimproving short-circuit resistance becomes low. When the average fiberlength is larger than the preferred range, it may be difficult to reducethe thickness of the substrate and cuttability may deteriorate.

The mass-weighted average fiber length of the fibrid is preferably 0.30to 1.00 mm. The length-weighted average fiber length of the fibrid ispreferably 0.10 to 0.50 mm. When the average fiber length is smallerthan the preferred range, the fibrid may drop off from the substrate.When the average fiber length is larger than the preferred range, thetextile of the substrate may become worse and the internal resistance ofthe substrate may become high.

When the fibrid has the above mass-weighted average fiber length and theabove length-weighted average fiber length, if the content of the fibridin the substrate is low, a fine network structure is formed by thefibers between fibrids and between fibrid and other fibers, whereby athin substrate having high tensile strength is easily obtained withoutimpairing cuttability.

When the fibrillated heat-resistant fibers (I) and (II) and the fibridhave the above mass-weighted average fiber lengths and the abovelength-weighted average fiber lengths, if the contents of theheat-resistance fibers in the substrate are low, a fine networkstructure is formed by the fibers between the heat-resistant fibers andbetween the heat-resistant fibers and the synthetic resin short fibers,whereby a substrate which has high tensile strength and can be made thinis easily obtained without impairing cuttability.

In the present invention, the mass-weighted average fiber length and thelength-weighted average fiber length are mass-weighted average fiberlength (L(w) and length-weighted average fiber length (L(l)) measured ina projection fiber length (Proj) mode with KajaaniFiberLab V3.5(manufactured by Metso Automation), respectively.

The average fiber width of the fibrillated heat-resistance fibers (I) ispreferably 0.5 to 20.0 μm, more preferably 3.0 to 16.0 μm, much morepreferably 5.0 to 15.0 μm. When the average fiber width is larger than20.0 μm, it may be difficult to reduce the thickness of the substrate orcuttability may deteriorate. When the average fiber width is smallerthan 0.5 μm, the fibrillated heat-resistant fibers (I) may drop off fromthe substrate.

The average fiber width of the fibrillated heat-resistant fibers (II) ispreferably 5.0 to 40.0 μm, more preferably 5.0 to 35.0 μm, much morepreferably 5.0 to 30.0 μm. When the average fiber width is larger than40.0 μm, it may be difficult to reduce the thickness of the substrate orcuttability may deteriorate. When the average fiber length is smallerthan 5.0 μm, short-circuit resistance may not improve.

The average fiber width of the fibrid is preferably 3.0 to 40.0 μm, morepreferably 5.0 to 35.0 μm, much more preferably 10.0 to 30.0 μm. Whenthe average fiber width is larger than 40.0 μm, the internal resistanceof the substrate tends to become high, it may be difficult to reduce thethickness, or cuttability may deteriorate. When the average fiber lengthis smaller than 3.0 μm, the fibrid beating time becomes long, therebygreatly reducing productivity.

In the present invention, the average fiber width is a fiber widthmeasured by using KajaaniFiberLab V.3.5 (manufactured by MetsoAutomation).

In Examples of the present invention, the modified freeness,mass-weighted average fiber length, length-weighted average fiber lengthand average fiber width of the heat-resistant fibers are the measurementvalues (A) of raw materials before the manufacture of the substrate. Inthe present invention, the measurement values (B) of the heat-resistantfibers taken out from the substrate were measured to check thedifferences between the measurement values (A) and the measurementvalues (B). A substrate containing polyethylene terephthalate-basedsynthetic resin short fibers and the fibrillated heat-resistant fibers(I) was put in an alkali-resistant plastic vessel. Then, ahigh-concentration alkali solution (for example, an 48 mass % potassiumhydroxide aqueous solution) was added to the vessel, and a stirrer wasinserted into the solution. Thereafter, the solution was stirred for oneday while the vessel was immersed in a 40 to 60° C. hot bath to dissolvethe PET-based synthetic resin short fibers completely. Then, thefibrillated heat-resistant fibers (I) in the vessel were filtered outwith a filter, neutralized, rinsed fully and dried to obtain thefibrillated heat-resistant fibers (I) taken out from the substrate. Themodified freeness and others of the fibrillated heat-resistant fibers(I) taken out from the substrate were measured as the measurement values(B). When the measurement values (A) before the manufacture of thesubstrate and the measurement values (B) were compared with each other,there were not so much differences between the measurement values (A)and (B), and it could be confirmed that measurement values (A)

measurement values (B).

<measurement values (A)> Modified freeness 50 ml Mass-weighted averagefiber length 0.58 mm Length-weighted average fiber length 0.33 mmAverage fiber width 15.5 μm

<measurement values (B)> Modified freeness 49 ml Mass-weighted averagefiber length 0.55 mm Length-weighted average fiber length 0.31 mmAverage fiber width 16.1 μm

In the present invention, the synthetic resin short fibers are shortfibers (staple) which are made of a synthetic resin such as polyolefin,polyester, polyvinyl acetate, ethylene-vinyl acetate copolymer,polyamide, acryl, polyvinyl chloride, polyvinylidene chloride, polyvinylether, polyvinyl ketone, polyether polyvinyl alcohol, diene,polyurethane, phenol, melamine, furan, urea, aniline, unsaturatedpolyester, fluorine, silicone or derivative thereof and are notfibrillated. The tensile strength and thrust strength of the substratecan be enhanced by containing the synthetic resin short fibers.

The synthetic resin short fibers may be fibers made of a single resin orcomposite fibers made of two or more resins. The synthetic resin shortfibers contained in the substrate of the present invention may be of onetype or two or more types. Examples of the composite fibers includecore-sheath, eccentric, side-by-side, sea-island, orange and multiplebimetal type composite fibers.

The fineness of the synthetic resin short fibers is preferably 0.01 to0.6 dtex, more preferably 0.02 to 0.3 dtex. When the fineness of thesynthetic resin short fibers is more than 0.6 dtex, the number of fibersin the thickness direction becomes small, whereby the pore sizedistribution of the substrate becomes wide with the result that aleakage current may become large. Further, it is difficult to reduce thethickness, and strength characteristic tends to deteriorate. When thefineness of the synthetic resin short fibers is less than 0.01 dtex, thefibers become very expensive, the stable production of the fibers may bedifficult, and dewaterability may deteriorate at the time of producingthe substrate by a wet papermaking method.

The fiber length of the synthetic short fibers is preferably 1 to 10 mm,more preferably 1 to 5 mm. When the fiber length is larger than 10 mm,the texture may become worse. When the fiber length is smaller than 1mm, the mechanical strength of the substrate lowers, whereby thesubstrate may be broken at the time of forming the inorganic particlelayer.

The substrate of the present invention may contain fibers other than theheat-resistant fibers and the synthetic resin short fibers. For example,cellulose fibers, pulp products and fibrillated products of cellulosefibers, pulp products of synthetic resins and inorganic fibers may beused. The inorganic fibers include glass, alumina, silica, ceramic andRockwool fibers. The cellulose fibers may be natural cellulose orregenerated cellulose fibers.

FIG. 4 is an electron photomicrograph of a substrate containing thefibrillated heat-resistant fibers (I) 1 as the heat-resistant fibers.FIG. 5 is an electron photomicrograph of a substrate containing thefibrillated heat-resistant fibers (I) 1 and the fibrillatedheat-resistant fibers (II) 2 as the heat-resistant fibers. FIG. 6 is anelectron photomicrograph of a substrate for lithium ion secondarybattery separators which contains the fibrillated heat-resistant fibers(I) 1 and the fibrid 3. FIG. 7 is an electron photomicrograph of asubstrate for lithium ion secondary battery separators which containsthe fibrillated heat-resistant fibers (I) 1, the fibrillatedheat-resistant fibers (II) 2 and the fibrid 3 as the heat-resistantfibers. FIG. 8 is an electron photomicrograph of a substrate containingno heat-resistant fibers. The fibrid 3 is thin leaf-like whereas thefibrillated heat-resistant fibers (I) 1 and (II) 2 are not thinleaf-like but microfibers which have been finely separated in adirection parallel mainly to the fiber axis. Since the fibrillatedheat-resistant fibers (I) 1 in particular are highly fibrillated, theyeasily get entangled with the synthetic resin short fibers 4 and areeasily spread into the entire substrate. Therefore, as described above,the substrate containing the fibrillated heat-resistant fibers (I) 1 asan essential component becomes the most excellent substrate which hasthe highest heat-resistant temperature and suppresses a heat flow.

The thickness of the substrate of the present invention is preferablynot smaller than 6 μm, more preferably not smaller than 8 μm, much morepreferably not smaller than 10 μm. Further, it is preferably not largerthan 20 μm, more preferably not larger than 18 μm, much more preferablynot larger than 15 μm. Even when the thickness of the substrate is setto the above range, the substrate of the present invention can reduceinternal resistance and retain tensile strength required for the coatingstep or the electrode lamination step, whereby work efficiency in eachstep including the papermaking property of the substrate is notimpaired. When the thickness of the substrate is larger than 20 μm, theresistance of the separator may become too high. Further, ahigh-capacity battery may not be obtained. When the thickness of thesubstrate is smaller than 6 μm, the strength of the substrate becomestoo low, whereby the substrate may be broken at the time of handling orcoating.

The density of the substrate of the present invention is preferably notlower than 0.40 g/cm³, more preferably not lower than 0.45 g/cm³, andpreferably not higher than 0.75 g/cm³, more preferably not higher than0.70 g/cm³. When the density is lower than 0.40 g/cm³, the strength ofthe substrate becomes too low, whereby the substrate may be broken atthe time of handling or coating. When the density is higher than 0.75g/cm³, the resistance of the separator may become too high.

The substrate of the present invention is preferably a wet nonwovenfabric produced by a wet paper-making method. The wet paper-makingmethod is to produce a wet nonwoven fabric by dispersing fibers in waterto obtain homogeneous paper-making slurry and filtering the paper-makingslurry with a paper machine. Examples of the paper machine include acylinder paper machine, fourdrinier paper machine, inclination typepaper machine, inclination short net paper machine and compositemachines thereof. In the process for producing a wet nonwoven fabric, awater-jet interlacing treatment may be carried out as required. As thetreatment of a wet nonwoven fabric, a heat treatment, calender or heatcalender may be carried out.

The lithium ion battery separator of the present invention isconstituted by combining at least one composite selected from the groupconsisting of an inorganic particle layer containing inorganicparticles, an organic particle layer containing organic particles, aporous film, a nanofiber layer, a solid electrolyte and a gel-likeelectrolyte with the substrate of the present invention. Out of these, aseparator having the substrate and the inorganic particle layer is mostpreferred.

The particle diameter of each of the inorganic particles is preferably0.02 to 4.00 μm, more preferably 0.10 to 3.00 μm. When the particlediameter is too large, it may be difficult to form an inorganic particlelayer having an appropriate thickness, or surface unevenness may becomelarge. When the particle diameter is too small, the inorganic particlesmay be hardly dispersed or easily drop off from the substrate, and theamount of a binder must be increased to prevent drop-off, therebydeteriorating internal resistance. The particle diameter as used hereinis the D50 (median diameter) of a volume distribution measured by alaser diffraction scattering method.

The inorganic particle layer may contain a binder. As the binder,various organic polymers may be used. The polymers include organicpolymers such as styrene-butadiene copolymer elastomer,acrylonitrile-butadiene copolymer elastomer, (meth)acrylic acid esterpolymer elastomer, styrene-(meth)acrylic acid ester polymer elastomerand polyvinylidene fluoride polymer.

The content of the binder in the inorganic particle layer is preferably2 to 200 mass % based on the total weight of the inorganic particles. Itis particularly preferably to 50 mass %. When the amount of the binderis too small, the inorganic particles may easily drop off from thesubstrate. When the amount of the binder is too large, the inorganicparticle layer becomes too fine, thereby reducing ion permeability.

The separator having the substrate and the inorganic particle layer canbe produced by forming the inorganic particle layer on at least one sideof the substrate. To form the inorganic particle layer on at least oneside of the substrate, there is a method in which slurry for forming theinorganic particle layer (coating solution) is prepared by dispersing ordissolving components constituting the inorganic particle layer in amedium such as water or an organic solvent and coated on the substrate.

As the medium for preparing the slurry for forming the inorganicparticle layer, any medium is acceptable if it can dissolve or dispersethe binder and the inorganic particles uniformly. For example, aromatichydrocarbons such as toluene, cyclic ethers such as tetrahydrofuran(THF), ketones such as methyl ethyl ketone (MEK), alcohols such asisopropyl alcohol, N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide(DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and watermay be used as required. These media may be mixed together as required.The medium in use is preferably a medium which does not swell ordissolve the substrate.

To apply the coating solution, for example, coating system such asblade, rod, reverse roll, lip, die, curtain or air knife coating system,printing system such as flexography, screen, offset, gravure or inkjetprinting system, or transfer system such as roll transfer or filmtransfer system may be selected and used as required.

As a device for applying the coating solution to the substrate to formthe inorganic particle layer, various coating devices may be used. Forexample, devices having coating system such as gravure, die, lip, blade,curtain, air knife, rod, roll, reverse roll coater, kiss or dip coatingsystem; printing system such as flexography, screen, offset, gravure orinkjet printing system, or transfer system such as roll transfer or filmtransfer system may be used.

The coating weight of the inorganic particle layer which depends on thesubstrate is preferably 1 to 30 g/m², more preferably 3 to 20 g/m², muchmore preferably 5 to 15 g/m² as a dried (absolute dried) coating weightper one side of the substrate. When the coating weight of the inorganicparticle layer is lower than 1 g/m², almost all the coating solutionpermeates the inside of the substrate at the time of forming theinorganic particle layer, whereby the inorganic particle layer may notbe formed on the surface of the substrate. When the coating weight ishigher than 30 g/m², the separator becomes thick, whereby the energydensity per volume of the battery becomes low and the basis weight ofthe separator becomes heavy, thereby reducing the energy density permass of the battery.

To the inorganic particle layer, various additives such as a dispersantexemplified by polyacrylic acid and sodium carboxymethyl cellulose,thickener exemplified by hydroxyethyl cellulose, sodium carboxymethylcellulose and polyethylene oxide, wetting agent, antiseptic anddefoaming agent may be added as required besides the above inorganicparticles and the binder. In general, a nonaqueous coating solutionprepared by using an organic solvent as a medium has low surface tensionwhereas an aqueous coating solution prepared by using water as a mediumhas high surface tension. Both the nonaqueous coating solution and theaqueous coating solution can be applied to the substrate of the presentinvention without a problem as the substrate has high coating solutionreceptivity.

A preferred example of the separator of the present invention having thesubstrate of the present invention and the inorganic particle layer is aseparator having an inorganic particle layer A containing magnesiumhydroxide having an average particle diameter of 2.0 to 4.0 μm and aninorganic particle layer B containing magnesium hydroxide having anaverage particle diameter of not less than 0.5 μm to less than 2.0 μmboth of which are formed on one side of the substrate of the presentinvention in the mentioned order. The inorganic particle layer A isobtained by applying a coating solution “a” containing magnesiumhydroxide having an average particle diameter of 2.0 to 4.0 μm to oneside of the substrate. The inorganic particle layer B is obtained byapplying a coating solution “b” containing magnesium hydroxide having anaverage particle diameter of not less than 0.5 μm to less than 2.0 μm tothe inorganic particle layer A.

The average particle diameter of magnesium hydroxide contained in theinorganic particle layer A is larger than the average particle diameterof magnesium hydroxide contained in the inorganic particle layer B.Therefore, the inorganic particle layer A hardly permeates the inside ofthe substrate and hardly produces a pinhole as compared with theinorganic particle layer B but tends to become rather thick. Theinorganic particle layer B easily permeates the inside of the substrateand produces a pinhole as compared with the inorganic particle layer Abut tends to become thin. Therefore, in the separator having theinorganic particle layer A and the inorganic particle layer B on oneside of the substrate in the mentioned order, the inorganic particlelayer A prevents the inorganic particle layer B from permeating theinside of the substrate, thereby making it possible to form thehomogeneous inorganic particle layers on the surface of the separator.As a result, the production of a pinhole can be prevented and internalresistance can be made low though the separator is thin.

Since the inorganic particle layer A easily adheres to the substrate andhas high layer strength, a defect is hardly produced at the time ofproducing the separator, thereby obtaining excellent productionstability.

Further, as the average particle diameter of magnesium hydroxidecontained in the inorganic particle layer A is larger than the averageparticle diameter of magnesium hydroxide contained in the inorganicparticle layer B, in the case of the same coating weight, the inorganicparticle layer A becomes thicker than the inorganic particle layer B.Therefore, in the case of separators having the same total coatingweight of the inorganic particle layer A and the inorganic particlelayer B, a separator having a lower coating weight of the inorganicparticle layer A than the coating weight of the inorganic particle layerB can be made thinner advantageously. When the coating weight of theinorganic particle layer A is higher than the coating weight of theinorganic particle layer B, the content of magnesium hydroxide having arelatively large average particle diameter becomes high, whereby theseparator may become thick.

Another preferred example of the separator of the present inventionhaving the substrate of the present invention and the inorganic particlelayers is a separator having an inorganic particle layer A containingmagnesium hydroxide having an average particle diameter of 2.0 to 4.0 μmon one side of the substrate of the present invention and an inorganicparticle layer B containing magnesium hydroxide having an averageparticle diameter of not less than 0.5 μm to less than 2.0 μm on theother side of the substrate. The inorganic particle layer A is obtainedby applying a coating solution “a” containing magnesium hydroxide havingan average particle diameter of 2.0 to 4.0 μm to one side of thesubstrate. The inorganic particle layer B is obtained by applying acoating solution “b” containing magnesium hydroxide having an averageparticle diameter of not less than 0.5 μm to less than 2.0 μm to theother side of the substrate opposite to the above side on which theinorganic particle layer A has been formed.

The average particle diameter of magnesium hydroxide contained in theinorganic particle layer A is larger than the average particle diameterof magnesium hydroxide contained in the inorganic particle layer B.Therefore, the inorganic particle layer A hardly permeates the inside ofthe substrate as compared with the inorganic particle layer B and hardlyproduces a pinhole but tends to become rather thick. The inorganicparticle layer B easily permeates the inside of the substrate andproduces a pinhole as compared with the inorganic particle layer A buttends to become thin. Then, by forming the inorganic particle layer Aand the inorganic particle layer B on different sides of the substrate,the production of a pinhole can be prevented and a thin separator havinglow internal resistance can be obtained.

Since the inorganic particle layer A and the inorganic particle layer Beasily adhere to the substrate and have high layer strength, a defect ishardly produced at the time of producing the separator, therebyobtaining excellent production stability.

Since the average particle dimeter of magnesium hydroxide contained inthe inorganic particle layer A is larger than the average particlediameter of magnesium hydroxide contained in the inorganic particlelayer B, in the case of the same coating weight, the inorganic particlelayer A is thicker than the inorganic particle layer B. Therefore, inthe case of separators having the same total coating weight of theinorganic particle layer A and the inorganic particle layer B, aseparator having a lower coating weight of the inorganic particle layerA than the coating weight of the inorganic particle layer B can be madethinner advantageously. When the coating weight of the inorganicparticle layer A is higher than the coating weight of the inorganicparticle layer B, the content of magnesium hydroxide having a relativelylarge average particle diameter becomes high, whereby the separator maybecome thick.

The average particle diameter of magnesium hydroxide contained in theinorganic particle layer A is preferably 2.2 to 3.7 μm, more preferably2.5 to 3.5 μm.

The dried coating weight of the inorganic particle layer A is preferably2.0 to 10.0 g/m², more preferably 2.5 to 8.0 g/m², much more preferably3.0 to 6.0 g/m². When the coating weight is higher than 10.0 g/m², theseparator may become too thick. When the coating weight is lower than2.0 g/m², a pinhole may be easily produced.

The average particle diameter of magnesium hydroxide contained in theinorganic particle layer B is preferably 0.5 to 1.5 μm, more preferably0.5 to 1.3 μm, particularly preferably 0.5 to 1.0 μm.

The dried coating weight of the inorganic particle layer B is preferably2.5 to 10.0 g/m², more preferably 3.0 to 8.5 g/m², much more preferably3.5 to 7.0 g/m². When the coating weight is higher than 10.0 g/m², theseparator may become too thick. When the coating weight is lower than2.5 g/m², a pinhole may be easily produced.

The dried coating weight ratio (dried coating weight of inorganicparticle layer A/dried coating weight of inorganic particle layer B) ofthe inorganic particle layer A and the inorganic particle layer B ispreferably not less than 0.23 to less than 1.00, more preferably notless than 0.33 to less than 0.95, much more preferably not less than0.43 to less than 0.90. When the dried coating weight ratio of theinorganic particle layer A and the inorganic particle layer B is notless than 1.00, the separator may become thick and when the driedcoating weight ratio is less than 0.23, a pinhole may be easilyproduced.

The basis weight of the separator of the present invention is preferably9 to 35 g/m², more preferably 10 to 30 g/m², much more preferably 12 to25 g/m². When the weight is higher than 35 g/m², internal resistance maybecome too high. When the weight is lower than 9 g/m², a pinhole may beeasily produced and sufficiently high strength may be hardly obtained.

The thickness of the separator of the present invention is preferably 9to 35 μm, more preferably 10 to 30 μm, much more preferably 12 to 28 μm.When the thickness is larger than 35 μm, the separator becomes too thickand internal resistance may become high. When the thickness is smallerthan 9 μm, a pinhole may be easily produced and sufficiently highstrength may be hardly obtained.

EXAMPLES

The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting. Percentage (%) and parts in Examples are all based on massunless otherwise noted. The coating weight is dried coating weight.

In

Example 1

to

Example 4

, the following physical properties of substrates for lithium ionbattery separators and lithium ion battery separators were measured andevaluated.

<Basis Weight of Substrate and Coating Weight of Inorganic ParticleLayer>

The basis weights of the substrates and the separators were measured inaccordance with JIS P8124. The coating weight of the inorganic particlelayer was calculated by subtracting the basis weight of the substratefrom the basis weight of the separator.

<Thicknesses of Substrate and Separator>

The thicknesses of the substrate and the separator were measured under aload of 5 N by using an external micrometer specified in JIS B7502.

<Adhesion Between Substrate and Inorganic Particle Layer>

Five test pieces measuring 100 mm in a width direction×100 mm in a flowdirection were cut out from each separator, and cellophane tapemeasuring 10 mm in width×50 mm in length was lightly attached to theinorganic particle layer of each of the separators in a flow directionand a width direction to reciprocate a 1 kg stainless cylinder twice.Thereafter, the cellophane tape was removed from the inorganic particlelayer, and the separator was visually checked and evaluated based on thefollowing criteria. Separators evaluated up to Δ can be actually used.

◯: inorganic particle layer remains on substrate surface

Δ: about half of inorganic particle layer peels off from substrate

X: almost all inorganic particle layer peels off from substrate

<Cuttability of Separator>

Each separator was slit into 8 rolls measuring 50 mm in a widthdirection×200 m in a flow direction, and both edge surfaces werevisually checked and evaluated based on the following criteria.Separators evaluated up to Δ can be actually used.

◯: both edge surfaces are not fluffed

Δ: both edge surfaces are slightly fluffed

X: both edge surfaces are markedly fluffed

<Manufacture of Battery for Evaluation>

A pouch type lithium ion battery for evaluation having a design capacityof 30 mAh was manufactured by using each separator, lithium manganate asa cathode, a mesocarbon microbead as an anode and a 1 mol/L diethylcarbonate/ethylene carbonate (volume ratio of 7/3) mixed solventsolution of lithium hexafluorophosphate as an electrolytic solution insuch a manner that the inorganic particle layer of each separator wasopposed to the anode and evaluated for the following internal resistanceand leakage current.

<Evaluation of Internal Resistance>

After five cycles of shake-down charging/discharging, each consisting of60 mA constant current charging, 4.2 V constant voltage charging (1hour), 60 mA constant current discharging and the next cycle when thevoltage became 2.8 V, were carried out for each battery, 60 mA constantcurrent charging, 4.2 V constant voltage charging (1 hour), 30 minutesof 6 mA constant current discharging (discharge amount of 3 mAh), themeasurement of voltage right before the end of discharging (voltage“a”), 60 mA constant current charging, 4.2 V constant voltage charging(1 hour), 2 minutes of 90 mA constant current discharging (dischargeamount of 3 mAh) and the measurement of voltage (voltage “b”) rightbefore the end of discharging were carried out to obtain internalresistance from the expression internal resistance Ω=(voltage“a”−voltage “b”)/(90 mA−6 mA). Separators evaluated up to Δ can beactually used.

⊚: internal resistance of less than 3.5Ω

◯: internal resistance of not less than 3.5Ω to less than 4.0Ω

Δ: internal resistance of not less than 4.0Ω to less than 5.0Ω

X: internal resistance of not less than 5.0Ω

<Evaluation of Leakage Current>

The charging capacity of each battery for evaluation was measured after30 mA constant current charging and 4.2 V constant voltage charging(termination current of 3 mA) were carried out for the first time. Eachseparator was classified into the following 4 levels according tocharging capacity. A charging capacity greatly exceeding 30 mAh which isa design capacity means that a leakage current is produced in thebattery. Batteries evaluated up to ◯ can be actually used.

⊚: first charging capacity of less than 32 mAh

◯: first charging capacity of not less than 32 mAh to less than 37 mAh

Δ: first charging capacity of not less than 37 mAh to less than 42 mAh

X: first charging capacity of not less than 42 mAh

<Evaluation of Tensile Strength>

A sample piece measuring 250 mm in a flow direction×50 mm in a widthdirection was cut out from each substrate in such a manner that its longside was arranged in a flow direction to carry out a tensile test at atensile speed of 200 mm/min by using a desk-top material testing machine(STA-1150 of ORIENTEC Co., Ltd.) in accordance with JIS P8113. Themaximum value of tensile stress is taken as tensile strength. Ingeneral, higher tensile strength is more preferred. When tensilestrength is low, tension applied to the substrate must be preciselycontrolled at the time of coating and manufacturing a battery, therebycausing a problem that a large-scaled apparatus is required for thiscontrol.

⊚: tensile strength of not less than 1,000 N/m

◯: tensile strength of not less than 700 N/m to less than 1,000 N/m

Δ: tensile strength of not less than 400 N/m to less than 700 N/m

X: tensile strength of less than 400 N/m

Example 1 Example 1-1

<Manufacture of Substrate>

55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)and 4.5 parts by mass of fibrillated heat-resistant fibers obtained byfibrillating a pulp-like product of wholly aromatic polyamide fibers(average fiber length of 1.7 mm, average fiber diameter of 10 μm) to amodified freeness of up to 50 ml with a high-pressure homogenizer weredispersed in water with a pulper to prepare homogeneous papermakingslurry having a concentration of 0.5 mass %, and a wet paper web wasobtained by using an inclination type paper machine and dried with acylinder drier having a surface temperature of 135° C. to obtain asheet. The obtained sheet was calendered with a heat calender having asteel roll plated with chromium and having a surface temperature of 195°C. as one of the rolls and a resin roll having a hardness of Shore A92as the other roll and a linear pressure of 100 kN/m to produce asubstrate 1-1 having a basis weight of 10 g/m² and a thickness of 15 μm.

<Preparation of Coating Solution>

100 parts of boehmite having a volume average particle diameter of 0.9μm and a specific surface area of 5.5 m²/g was mixed with 120 parts of a0.3% aqueous solution of a carboxymethyl cellulose sodium salt having aviscosity at 25° C. of a 1 mass % solution thereof of 200 mPa·s andfully stirred, and then the resulting solution was mixed with 300 partsof a 0.5% aqueous solution of a carboxymethyl cellulose sodium salthaving a viscosity at 25° C. of a 1 mass % aqueous solution thereof of7,000 mPa·s and 10 parts of a commercially available styrene butadienerubber (SBR)-based binder for lithium ion batteries (TRD2001 of JSRCorporation) (solid concentration of 48%) and stirred to prepare acoating solution.

<Manufacture of Separator>

The coating solution was applied to one side of the above substrate 1-1with a kiss reverse gravure coater to a coating weight of 12 g/m² anddried to manufacture a separator having an inorganic particle layer.

Example 1-2

A substrate 1-2 having a basis weight of 10 g/m² and a thickness of 15μm was manufactured in the same manner as the substrate 1-1 except that59.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm and 1.0 part by mass of fibrillated heat-resistant fibershaving a modified freeness of 50 ml were used. Then, the coatingsolution prepared in Example 1-1 was applied to a coating weight of 12g/m² in the same manner as in Example 1-1 and dried to manufacture aseparator.

Example 1-3

A substrate 1-3 having a basis weight of 8 g/m² and a thickness of 11 μmwas manufactured by using the papermaking slurry of Example 1-1 in thesame manner as the substrate 1-1. Then, the coating solution prepared inExample 1-1 was applied to a coating weight of 12 g/m² in the samemanner as in Example 1-1 and dried to manufacture a separator.

Example 1-4

A substrate 1-4 having a basis weight of 8 g/m² and a thickness of 11 μmwas manufactured in the same manner as the substrate 1-1 except that55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)and 4.5 parts by mass of fibrillated heat-resistant fibers obtained byfibrillating cut fibers (average fiber length of 3.0 mm, average fiberdiameter of 20 μm) of polyarylate fibers (wholly aromatic polyesterfibers) to a modified freeness of up to 250 ml with a high-pressurehomogenizer were used. Then, the coating solution prepared in Example1-1 was applied to a coating weight of 12 g/m² in the same manner as inExample 1-1 and dried to manufacture a separator.

Comparative Example 1-1

A substrate 1-5 having a basis weight of 8 g/m² and a thickness of 11 μmwas manufactured in the same manner as the substrate 1-1 except that60.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mmand 40.0 parts by mass of undrawn PET-based synthetic resin short fibersfor single-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)were used. Then, the coating solution prepared in Example 1-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example1-1 and dried to manufacture a separator.

Comparative Example 1-2

A substrate 1-6 having a basis weight of 8 g/m² and a thickness of 11 μmwas manufactured in the same manner as the substrate 1-1 except that55.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)and 5.0 parts by mass of fibrillated heat-resistant fibers having amodified freeness of 50 ml were used and heat calender was carried outat a steel roll surface temperature of 195° C. and a linear pressure of120 kN/m to make the substrate uniform in thickness. Then, the coatingsolution prepared in Example 1-1 was applied to a coating weight of 12g/m² in the same manner as in Example 1-1 and dried to manufacture aseparator.

Comparative Example 1-3

A substrate 1-7 having a basis weight of 8 g/m² and a thickness of 11 μmwas manufactured in the same manner as the substrate 1-1 except that50.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)and 10.0 parts by mass of fibrillated heat-resistant fibers having amodified freeness of 50 ml were used and heat calender was carried outat a steel roll surface temperature of 195° C. and a linear pressure of175 kN/m to make the substrate uniform in thickness. Then, the coatingsolution prepared in Example 1-1 was applied to a coating weight of 12g/m² in the same manner as in Example 1-1 and dried to manufacture aseparator.

Comparative Example 1-4

A substrate 1-8 having a basis weight of 8 g/m² and a thickness of 11 μmwas manufactured in the same manner as the substrate 1-1 except thatwholly aromatic polyamide fibers which had been fibrillated to amodified freeness of up to 320 ml by using a grinding apparatus wereused in place of the fibrillated heat-resistant fibers. Then, thecoating solution prepared in Example 1-1 was applied to a coating weightof 12 g/m² in the same manner as in Example 1-1 and dried to manufacturea separator.

Example 1-5

A substrate 1-9 having a basis weight of 7 g/m² and a thickness of 10 μmwas manufactured by using the papermaking slurry of Example 1-1 in thesame manner as the substrate 1-1. Then, the coating solution prepared inExample 1-1 was applied to a coating weight of 12 g/m² in the samemanner as in Example 1-1 and dried to manufacture a separator.

Comparative Example 1-5

A substrate 1-10 having a basis weight of 7 g/m² and a thickness of 10μm was manufactured by using the papermaking slurry of ComparativeExample 1-1 in the same manner as the substrate 1-1. Then, the coatingsolution prepared in Example 1-1 was applied to a coating weight of 12g/m² in the same manner as in Example 1-1 and dried to manufacture aseparator.

Example 1-6

A substrate 1-11 having a basis weight of 8 g/m² and a thickness of 11μm was manufactured in the same manner as the substrate 1-1 except that50.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 4.5 parts by mass of fibrillated heat-resistant fibershaving a modified freeness of 50 ml and 5.5 pars by mass of solvent spuncellulose fibers obtained by refining solvent spun cellulose fibershaving an average fiber diameter of 10 μm and a fiber length of 4 mm byusing a refiner and beating them to a modified freeness of 90 ml wereused. Then, the coating solution prepared in Example 1-1 was applied toa coating weight of 12 g/m² in the same manner as in Example 1-1 anddried to manufacture a separator.

Example 1-7

A substrate 1-12 having a basis weight of 8 g/m² and a thickness of 11μm was manufactured in the same manner as the substrate 1-11 except that49.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 4.5 parts by mass of fibrillated heat-resistant fibershaving a modified freeness of 50 ml and 6.5 pars by mass of solvent spuncellulose fibers which were beaten to a modified freeness of 90 ml wereused. Then, the coating solution prepared in Example 1-1 was applied toa coating weight of 12 g/m² in the same manner as in Example 1-1 anddried to manufacture a separator.

Example 1-8

A substrate 1-13 having a basis weight of 7 g/m² and a thickness of 10μm was manufactured in the same manner as the substrate 1-1 except that56.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm and 4.0 parts by mass of fibrillated heat-resistantfibers having a modified freeness of 50 ml were used. Then, the coatingsolution prepared in Example 1-1 was applied to a coating weight of 12g/m² in the same manner as in Example 1-1 and dried to manufacture aseparator.

Example 1-9

A substrate 1-14 having a basis weight of 7 g/m² and a thickness of 10μm was manufactured in the same manner as the substrate 1-1 except that57.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm and 3.0 parts by mass of fibrillated heat-resistantfibers having a modified freeness of 50 ml were used. Then, the coatingsolution prepared in Example 1-1 was applied to a coating weight of 12g/m² in the same manner as in Example 1-1 and dried to manufacture aseparator.

Example 1-10

A substrate 1-15 having a basis weight of 12 g/m² and a thickness of 18μm was manufactured by using the papermaking slurry of Example 1-8 inthe same manner as the substrate 1-13. Then, the coating solutionprepared in Example 1-1 was applied to a coating weight of 15 g/m² inthe same manner as in Example 1-1 and dried to manufacture a separator.

Comparative Example 1-6

A substrate 1-16 having a basis weight of 12 g/m² and a thickness of 18μm was manufactured in the same manner as the substrate 1-7 except thatheat calender was carried out by using the papermaking slurry ofComparative Example 1-3 at a steel roll surface temperature of 195° C.and a linear pressure of 120 kN/m to make the substrate uniform inthickness. Then, the coating solution prepared in Example 1-1 wasapplied to a coating weight of 15 g/m² in the same manner as in Example1-1 and dried to manufacture a separator.

TABLE 1-1 Ex. 1-1 Ex. 1-2 Ex. 1-3 Ex. 1-4 Ex. 1-5 Sub. 1-1 Sub. 1-2 Sub.1-3 Sub. 1-4 Sub. 1-9 Fiber blending oriented crystallized pbm 55.5 59.055.5 55.5 55.5 PET-based synthetic resin short fibers 0.06 dtex × 3 mmUndrawn PET-based synthetic pbm 40.0 40.0 40.0 40.0 40.0 resin shortfibers 0.2 dtex × 3 mm Fibrillated heat-resistant pbm 4.5 1.0 4.5 4.54.5 fibers Beaten melt spun cellulose pbm 0.0 0.0 0.0 0.0 0.0 fibersFibrillated Modified freeness ml 50 50 50 250 50 heat-resistantMass-weighted average fiber mm 0.58 0.58 0.58 0.82 0.58 fibers lengthLength-weighted average mm 0.33 0.33 0.33 0.45 0.33 fiber length Fiberwidth μm 15.5 15.5 15.5 19.5 15.5 Linear pressure of heat calender kN/m100 100 100 100 100 Physical Basis weight of substrate g/m² 10 10 8 8 7properties Thickness of substrate μm 15 15 11 11 10 Coating weight ofcoating g/m² 12 12 12 12 12 layer Thickness of separator μm 25 25 21 2120 Evaluation Adhesion between substrate — ◯ ◯ ◯ ◯ ◯ and coating layerCuttability of separator — ◯ ◯ ◯ ◯ ◯ Internal resistance — ⊚ ⊚ ⊚ ◯ ⊚Leakage current — ⊚ ◯ ⊚ ◯ ⊚ Tensile strength — ⊚ ⊚ ◯ ◯ ◯ Ex. 1-6 Ex. 1-7Ex. 1-8 Ex. 1-9 Ex. 1-10 Sub. 1-11 Sub. 1-12 Sub. 1-13 Sub. 1-14 Sub.1-15 Fiber blending oriented crystallized pbm 50.0 49.0 56.0 57.0 56.0PET-based synthetic resin short fibers 0.06 dtex × 3 mm UndrawnPET-based synthetic pbm 40.0 40.0 40.0 40.0 40.0 resin short fibers 0.2dtex × 3 mm Fibrillated heat-resistant pbm 4.5 4.5 4.0 3.0 4.0 fibersBeaten melt spun cellulose pbm 5.5 6.5 0.0 0.0 0.0 fibers FibrillatedModified freeness ml 50 50 50 50 50 heat-resistant Mass-weighted averagefiber mm 0.58 0.58 0.58 0.58 0.58 fibers length Length-weighted averagemm 0.33 0.33 0.33 0.33 0.33 fiber length Fiber width μm 15.5 15.5 15.515.5 15.5 Linear pressure of heat calender kN/m 100 100 100 100 100Physical Basis weight of substrate g/m² 8 8 7 7 12 properties Thicknessof substrate μm 11 11 10 10 18 Coating weight of coating g/m² 12 12 1212 15 layer Thickness of separator μm 21 21 20 20 30 Evaluation Adhesionbetween substrate — ◯ ◯ ◯ ◯ ◯ and coating layer Cuttability of separator— ◯ ◯ ◯ ◯ ◯ Internal resistance — ⊚ ⊚ ⊚ ⊚ ◯ Leakage current — ⊚ ⊚ ⊚ ⊚ ⊚Tensile strength — ◯ Δ ◯ ◯ ⊚ Ex.: Example Sub.: Substrate pbm: part bymass

TABLE 1-2 C. Ex. 1-1 C. Ex. 1-2 C. Ex. 1-3 C. Ex. 1-4 C. Ex. 1-5 C. Ex.1-6 Sub. 1-5 Sub. 1-6 Sub. 1-7 Sub. 1-8 Sub. 1-10 Sub. 1-16 Fiberoriented crystallized pbm 60.0 55.0 50.0 55.5 60.0 50.0 blendingPET-based synthetic resin short fibers 0.06 dtex × 3 mm UndrawnPET-based synthetic pbm 40.0 40.0 40.0 40.0 40.0 40.0 resin short fibers0.2 dtex × 3 mm Fibrillated heat-resistant pbm 0.0 5.0 10.0 4.5 0.0 10.0fibers Beaten melt spun cellulose pbm 0.0 0.0 0.0 0.0 0.0 0.0 fibersFibrillated Modified freeness ml — 50 50 320 — 50 heat-resistantMass-weighted average fiber mm — 0.58 0.58 1.30 — 0.58 fibers lengthLength-weighted average mm — 0.33 0.33 0.59 — 0.33 fiber length Fiberwidth μm — 15.5 15.5 25.2 — 15.5 Linear pressure of heat calender kN/m100 120 175 100 100 120 Physical Basis weight of substrate g/m² 8 8 8 87 12 properties Thickness of substrate μm 11 11 11 11 10 18 Coatingweight of g/m² 12 12 12 12 12 15 coating layer Thickness of separator μm21 21 21 21 20 30 Evaluation Adhesion between substrate — ◯ Δ Δ ◯ ◯ Δand coating layer Cuttability of separator — ◯ Δ X Δ ◯ X Internalresistance — ⊚ Δ X ◯ ⊚ ◯ Leakage current — Δ ⊚ ⊚ Δ X ⊚ Tensile strength— ◯ ◯ Δ ◯ ◯ ◯ C. Ex.: Comparative Example Sub.: Substrate pbm: part bymass

The substrates manufactured in Examples 1-1 to 1-10 contained thefibrillated heat-resistant fibers (I) and the synthetic resin shortfibers, and the content of the fibrillated heat-resistant fibers (I) wasnot less than 1.0 mass to less than 5.0 mass % based on the total of allthe fiber components contained in the substrate. The separators havingthe respective substrates of Examples 1-1 to 1-10 and the inorganicparticle layer had high adhesion between the substrate and the inorganicparticle layer and excellent cuttability. Further, they had low internalresistance and little leakage current, and especially the separators ofExamples 1-1 to 1-9 had excellent strength characteristic though thesubstrates had a small thickness of not more than 15 μm.

The substrates of Examples 1-6 and 1-7 contained fibers other than thefibrillated heat-resistant fibers (I) and the synthetic resin shortfibers. From comparison between Example 1-6 and Example 1-7, thesubstrate of Example 1-6 having a synthetic resin short fiber content ofnot less than 90.0 mass % was superior in strength characteristic.

Since the substrates of Comparative Examples 1-1 and 1-5 did not containthe fibrillated heat-resistant fibers (I), when the basis weight of thesubstrate was reduced, a leakage current was apt to be produced.

The substrates of Comparative Examples 1-2 and 1-3 contained thefibrillated heat-resistant fibers (I) and the synthetic resin shortfibers, and the content of the fibrillated heat-resistant fibers (I) wasnot less than 5.0 mass % based on the total of all the fiber componentscontained in the substrate. Therefore, as the coating solution hardlypermeated the substrate, adhesion between the substrate and theinorganic particle layer lowered. In the evaluation of cuttability, alarge amount of fluff remained on the edge surfaces. Further, it wasfound that the linear pressure had to be raised to control the thicknessof the substrate to a predetermined value by heat calender, therebymaking it difficult to reduce the thickness of the substrate. As aresult, the internal resistance of the battery deteriorated. InComparative Example 1-3, the substrate creased at the time of coating astensile strength also lowered, and the separator shrank in width or wasbroken as tension could not be controlled in the step of laminating anelectrode.

Since the substrate of Comparative Example 1-4 did not contain thefibrillated heat-resistant fibers (I) and contained only the fibrillatedheat-resistant fibers (II), cuttability was low and fluff was slightlyseen on the edge surfaces. The leakage current deteriorated.

Although the substrates of Examples 1-10 and Comparative Example 1-6 hada thickness of 18 μm, the content of the fibrillated heat-resistantfibers (I) was 4.0 mass % in the substrate of Example 1-10 and 10.0 mass% in the substrate of Comparative Example 1-6. In the case of a thicksubstrate, even when the content of the fibrillated heat-resistantfibers (I) was not less than 5.0 mass %, internal resistance was low.Although the substrates of Example 1-8 and Comparative Example 1-3 had athickness of to 11 μm, the content of the fibrillated heat-resistantfibers (I) was 4.0 mass % in the substrate of Example 1-8 and 10.0 mass% in the substrate of Comparative Example 1-3. In the case of a thinsubstrate, as the content of the fibrillated heat-resistant fibers (I)was less than 5.0 mass %, internal resistance became low.

Example 2 Example 2-1

<Manufacture of Substrate>

55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.),1.0 part by mass of fibrillated heat-resistant fibers obtained byfibrillating a pulp-like product of wholly aromatic polyamide fibers(average fiber length of 1.7 mm, average fiber diameter of 10 μm) to amodified freeness of up to 350 ml with a high-pressure homogenizer and3.5 parts by mass of fibrillated heat-resistant fibers obtained byfibrillating a pulp-like product of wholly aromatic polyamide fibers toa modified freeness of up to 50 ml with a high-pressure homogenizer weredispersed in water with a pulper to prepare homogeneous papermakingslurry having a concentration of 0.5 mass %, and a wet paper web wasobtained by using an inclination type paper machine and dried with acylinder drier having a surface temperature of 135° C. to obtain asheet. The obtained sheet was calendered with a heat calender having asteel roll plated with chromium and having a surface temperature of 195°C. as one of the rolls and a resin roll having a hardness of Shore A92as the other roll and a linear pressure of 100 kN/m to manufacture asubstrate 2-1 having a basis weight of 10 g/m² and a thickness of 15 μm.

<Preparation of Coating Solution>

100 parts of boehmite having a volume average particle diameter of 0.9μm and a specific surface area of 5.5 m²/g was mixed with 120 parts of a0.3% aqueous solution of a carboxymethyl cellulose sodium salt having aviscosity at 25° C. of a 1 mass % aqueous solution thereof of 200 mPa·sand fully stirred, and then the resulting solution was mixed with 300parts of a 0.5% aqueous solution of a carboxymethyl cellulose sodiumsalt having a viscosity at 25° C. of a 1 mass % aqueous solution thereofof 7,000 mPa·s and 10 parts of a commercially available styrenebutadiene rubber (SBR)-based binder for lithium ion batteries (TRD2001of JSR Corporation) (solid concentration of 48%) and stirred to preparea coating solution.

<Manufacture of Separator>

The coating solution was applied to one side of the above substrate 2-1with a kiss reverse gravure coater to a coating weight of 12 g/m² anddried to manufacture a separator having an inorganic particle layer.

Example 2-2

A substrate 2-2 having a basis weight of 10 g/m² and a thickness of 15μm was manufactured in the same manner as the substrate 2-1 except that55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm, 3.5parts by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml and 1.0 part by mass of fibrillated whollyaromatic polyamide fibers having a modified freeness of 50 ml were used.Then, the coating solution prepared in Example 2-1 was applied to acoating weight of 12 g/m² in the same manner as in Example 2-1 and driedto manufacture a separator.

Example 2-3

A substrate 2-3 having a basis weight of 10 g/m² and a thickness of 15μm was manufactured in the same manner as the substrate 2-1 except that58.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm, 1.0part by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml and 1.0 part by mass of fibrillated whollyaromatic polyamide fibers having a modified freeness of 50 ml were used.Then, the coating solution prepared in Example 2-1 was applied to acoating weight of 12 g/m² in the same manner as in Example 2-1 and driedto manufacture a separator.

Example 2-4

A substrate 2-4 having a basis weight of 9 g/m² and a thickness of 14 μmwas manufactured in the same manner as the substrate 2-1 except that54.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.5parts by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml and 3.5 parts by mass of fibrillated whollyaromatic polyamide fibers having a modified freeness of 50 ml were used.Then, the coating solution prepared in Example 2-1 was applied to acoating weight of 12 g/m² in the same manner as in Example 2-1 and driedto manufacture a separator.

Example 2-5

A substrate 2-5 having a basis weight of 7 g/m² and a thickness of 11 μmwas manufactured in the same manner as the substrate 2-1 except that51.2 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm, 3.9parts by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml and 4.9 parts by mass of fibrillated whollyaromatic polyamide fibers having a modified freeness of 50 ml were usedand heat calender was carried out at a steel roll surface temperature of195° C. and a linear pressure of 110 kN/m to make the substrate uniformin thickness. Then, the coating solution prepared in Example 2-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example2-1 and dried to manufacture a separator.

Example 2-6

A substrate 2-6 having a basis weight of 7 g/m² and a thickness of 11 μmwas manufactured in the same manner as the substrate 2-1 except that54.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.5parts by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml and 3.5 parts by mass of fibrillatedpolyarylate fibers obtained by fibrillating cut fibers (average fiberlength of 3.0 mm, average fiber diameter of 20 μm) of polyarylate fibers(wholly aromatic polyester fibers) to a modified freeness of up to 250ml with a high-pressure homogenizer were used. Then, the coatingsolution prepared in Example 2-1 was applied to a coating weight of 12g/m² in the same manner as in Example 2-1 and dried to manufacture aseparator.

Comparative Example 2-1

A substrate 2-7 having a basis weight of 7 g/m² and a thickness of 10 μmwas manufactured in the same manner as the substrate 2-1 except that60.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mmand 40.0 parts by mass of undrawn PET-based synthetic resin short fibersfor binders having a fineness of 0.2 dtex and a fiber length of 3 mmwere used. Then, the coating solution prepared in Example 2-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example2-1 and dried to manufacture a separator.

Example 2-7

A substrate 2-8 having a basis weight of 7 g/m² and a thickness of 11 μmwas manufactured in the same manner as the substrate 2-1 except that52.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm, 2.5parts by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml and 5.0 parts by mass of fibrillated whollyaromatic polyamide fibers having a modified freeness of 50 ml were usedand heat calender was carried out at a steel roll surface temperature of195° C. and a linear pressure of 120 kN/m to make the substrate uniformin thickness. Then, the coating solution prepared in Example 2-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example2-1 and dried to manufacture a separator.

Example 2-8

A substrate 2-9 having a basis weight of 7 g/m² and a thickness of 11 μmwas manufactured in the same manner as the substrate 2-1 except that52.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm, 4.0parts by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml and 3.5 parts by mass of fibrillatedheat-resistant fibers having a modified freeness of 50 ml were used andheat calender was carried out at a steel roll surface temperature of195° C. and a linear pressure of 125 kN/m to make the substrate uniformin thickness. Then, the coating solution prepared in Example 2-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example2-1 and dried to manufacture a separator.

Example 2-9

A substrate 2-10 having a basis weight of 7 g/m² and a thickness of 11μm was manufactured in the same manner as the substrate 2-1 except that50.8 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm, 4.1parts by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml and 5.1 parts by mass of fibrillatedheat-resistant fibers having a modified freeness of 50 ml were used andheat calender was carried out at a steel roll surface temperature of195° C. and a linear pressure of 135 kN/m to make the substrate uniformin thickness. Then, the coating solution prepared in Example 2-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example2-1 and dried to manufacture a separator.

Example 2-10

A substrate 2-11 having a basis weight of 7 g/m² and a thickness of 11μm was manufactured in the same manner as the substrate 2-1 except that50.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm, 3.5parts by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml, 4.5 parts by mass of fibrillated whollyaromatic polyamide fibers having a modified freeness of 50 ml and 2.0parts by mass of solvent spun cellulose fibers obtained by refiningsolvent spun cellulose fibers having an average fiber diameter of 10 μmand a fiber length of 4 mm with a refiner and beating to a modifiedfreeness of 90 ml were used. Then, the coating solution prepared inExample 2-1 was applied to a coating weight of 12 g/m² in the samemanner as in Example 2-1 and dried to manufacture a separator.

Example 2-11

A substrate 2-13 having a basis weight of 7 g/m² and a thickness of 11μm was manufactured in the same manner as the substrate 2-1 except that49.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm, 3.5parts by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml, 4.5 parts by mass of fibrillated whollyaromatic polyamide fibers having a modified freeness of 50 ml and 3.0parts by mass of solvent spun cellulose fibers obtained by refiningsolvent spun cellulose fibers having an average fiber diameter of 10 μmand a fiber length of 4 mm with a refiner and beating to a modifiedfreeness of 90 ml were used. Then, the coating solution prepared inExample 2-1 was applied to a coating weight of 12 g/m² in the samemanner as in Example 2-1 and dried to manufacture a separator.

Example 2-12

A substrate 2-14 having a basis weight of 6 g/m² and a thickness of 10μm was manufactured by using the papermaking slurry of Example 2-1 inthe same manner as the substrate 2-1. Then, the coating solutionprepared in Example 2-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 2-1 and dried to manufacture a separator.

Comparative Example 2-2

A substrate 2-12 having a basis weight of 6 g/m² and a thickness of 10μm was manufactured in the same manner as the substrate 2-1 except thatheat calender was carried out by using the papermaking slurry ofComparative Example 2-1 at a steel roll surface temperature of 195° C.and a linear pressure of 90 kN/m to make the substrate uniform inthickness. Then, the coating solution prepared in Example 2-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example2-1 and dried to manufacture a separator.

Comparative Example 2-3

A substrate 2-15 having a basis weight of 7 g/m² and a thickness of 11μm was manufactured in the same manner as the substrate 2-1 except that51.1 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm and 8.9parts by mass of fibrillated wholly aromatic polyamide fibers having amodified freeness of 350 ml were used and heat calender was carried outat a steel roll surface temperature of 195° C. and a linear pressure of175 kN/m to make the substrate uniform in thickness. Then, the coatingsolution prepared in Example 2-1 was applied to a coating weight of 12g/m² in the same manner as in Example 2-1 and dried to manufacture aseparator.

Comparative Example 2-4

A substrate 2-16 having a basis weight of 7 g/m² and a thickness of 11μm was manufactured in the same manner as the substrate 2-1 except that51.1 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forbinders having a fineness of 0.2 dtex and a fiber length of 3 mm and 8.9parts by mass of fibrillated heat-resistant fibers having a modifiedfreeness of 50 ml were used and heat calender was carried out at a steelroll surface temperature of 195° C. and a linear pressure of 160 kN/m tomake the substrate uniform in thickness. Then, the coating solutionprepared in Example 2-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 2-1 and dried to manufacture a separator.

TABLE 2-1 Ex. 2-1 Ex. 2-2 Ex. 2-3 Ex. 2-4 Ex. 2-5 Ex. 2-6 Sub. 2-1 Sub.2-2 Sub. 2-3 Sub. 2-4 Sub. 2-5 Sub. 2-6 Fiber oriented crystallized Pbm55.5 55.5 58.0 54.0 51.2 54.0 blending PET-based synthetic resin shortfibers 0.06 dtex × 3 mm Undrawn PET-based synthetic Pbm 40.0 40.0 40.040.0 40.0 40.0 resin short fibers 0.2 dtex × 3 mm Fibrillatedheat-resistant Pbm 3.5 1.0 1.0 3.5 4.9 3.5 fibers (I) Fibrillatedheat-resistant Pbm 1.0 3.5 1.0 2.5 3.9 2.5 fibers (II) Beaten melt spuncellulose Pbm 0.0 0.0 0.0 0.0 0.0 0.0 fibers Fibrillated Modifiedfreeness ml 50 50 50 50 50 250 heat-resistant Mass-weighted averagefiber mm 0.58 0.58 0.58 0.58 0.58 0.82 fibers (I) length Length-weightedaverage mm 0.33 0.33 0.33 0.33 0.33 0.45 fiber length Fiber width μm15.5 15.5 15.5 15.5 15.5 19.5 Fibrillated Modified freeness ml 350 350350 350 350 350 heat-resistant Mass-weighted average mm 1.30 1.30 1.301.30 1.30 1.30 fibers (II) fiber length Length-weighted mm 0.59 0.590.59 0.59 0.59 0.59 average fiber length Fiber width μm 25.2 25.2 25.225.2 25.2 25.2 Linear pressure of heat calender kN/m 100 100 100 100 110100 Physical Basis weight of substrate g/m² 10 10 10 9 7 7 propertiesThickness of substrate μm 15 15 15 14 11 11 Coating weight of coatingg/m² 12 12 12 12 12 12 layer Thickness of separator μm 25 25 25 24 21 21Evaluation Adhesion between substrate — ◯ ◯ ◯ ◯ ◯ ◯ and coating layerCuttability of separator — ◯ ◯ ◯ ◯ ◯ ◯ Internal resistance — ⊚ ⊚ ⊚ ◯ ◯ ◯Leakage current — ⊚ ⊚ ⊚ ⊚ ⊚ ◯ Tensile strength — ⊚ ⊚ ⊚ ◯ ◯ ◯ Ex. 2-7 Ex.2-8 Ex. 2-9 Ex. 2-10 Ex. 2-11 Ex. 2-12 Sub. 2-8 Sub. 2-9 Sub. 2-10 Sub.2-11 Sub. 2-13 Sub. 2-14 Fiber oriented crystallized pbm 52.5 52.5 50.850.0 49.0 55.5 Blending PET-based synthetic resin short fibers 0.06 dtex× 3 mm Undrawn PET-based synthetic pbm 40.0 40.0 40.0 40.0 40.0 40.0resin short fibers 0.2 dtex × 3 mm Fibrillated heat-resistant pbm 5.03.5 5.1 4.5 4.5 3.5 fibers (I) Fibrillated heat-resistant pbm 2.5 4.04.1 3.5 3.5 1.0 fibers (II) Beaten melt spun cellulose pbm 0.0 0.0 0.02.0 3.0 0.0 fibers Fibrillated Modified freeness ml 50 50 50 50 50 50heat-resistant Mass-weighted average fiber mm 0.58 0.58 0.58 0.58 0.580.58 fibers (I) length Length-weighted average mm 0.33 0.33 0.33 0.330.33 0.33 fiber length Fiber width μm 15.5 15.5 15.5 15.5 15.5 15.5Fibrillated Modified freeness ml 350 350 350 350 350 350 heat-resistantMass-weighted average mm 1.30 1.30 1.30 1.30 1.30 1.30 fibers (II) fiberlength Length-weighted average mm 0.59 0.59 0.59 0.59 0.59 0.59 fiberlength Fiber width μm 25.2 25.2 25.2 25.2 25.2 25.2 Linear pressure ofheat calender kN/m 120 125 135 100 100 100 Physical Basis weight ofsubstrate g/m² 7 7 7 7 7 6 properties Thickness of substrate μm 11 11 1111 11 10 Coating weight of coating g/m² 12 12 12 12 12 12 layerThickness of separator μm 21 21 21 21 21 20 Evaluation Adhesion betweensubstrate — Δ Δ Δ ◯ ◯ ◯ and coating layer Cuttability of separator — Δ ΔΔ ◯ ◯ ◯ Internal resistance — Δ Δ Δ ◯ ◯ ⊚ Leakage current — ⊚ ⊚ ⊚ ⊚ ⊚ ⊚Tensile strength — ◯ ◯ Δ ◯ Δ ◯ Ex.: Example Sub.: Substrate pbm: part bymass

TABLE 2-2 C. Ex. 2-1 C. Ex. 2-2 C. Ex. 2-3 C. Ex. 2-4 Sub. 2-7 Sub. 2-12Sub. 2-15 Sub. 2-16 Fiber oriented crystallized pbm 60.0 60.0 51.1 51.1blending PET-based synthetic resin short fibers 0.06 dtex × 3 mm UndrawnPET-based synthetic pbm 40.0 40.0 40.0 40.0 resin short fibers 0.2 dtex× 3 mm Fibrillated heat-resistant pbm 0.0 0.0 0.0 8.9 fibers (I)Fibrillated heat-resistant pbm 0.0 0.0 8.9 0.0 fibers (II) Beaten meltspun cellulose pbm 0.0 0.0 0.0 0.0 fibers Fibrillated Modified freenessml — — — 50 heat-resistant Mass-weighted average fiber mm — — — 0.58fibers (I) length Length-weighted average mm — — — 0.33 fiber lengthFiber width μm — — — 15.5 Fibrillated Modified freeness ml — — 350 —heat-resistant Mass-weighted average fiber mm — — 1.30 — fibers (II)length Length-weighted average mm — — 0.59 — fiber length Fiber width μm— — 25.2 — Linear pressure of heat calender kN/m 100 90 175 160 PhysicalBasis weight of substrate g/m² 7 6 7 7 properties Thickness of substrateμm 10 10 11 11 Coating weight of coating g/m² 12 12 12 12 layerThickness of separator μm 20 20 20 20 Evaluation Adhesion betweensubstrate — ◯ ◯ X Δ and coating layer Cuttability of separator — ◯ ◯ X XInternal resistance — ⊚ ⊚ X X Leakage current — X X ◯ ⊚ Tensile strength— ◯ ◯ Δ Δ C. Ex.: Comparative Example Sub.: Substrate pbm: part by mass

The substrates manufactured in Examples 2-1 to 2-12 contained thefibrillated heat-resistant fibers (I), the fibrillated heat-resistantfibers (II) and the synthetic resin short fibers, had high adhesion tothe inorganic particle layer and was excellent in cuttability. Further,internal resistance was low, a leakage current was little, and strengthcharacteristic was excellent.

Out of the substrates manufactured in Examples 2-1 to 2-12, thesubstrates of Examples 2-1 to 2-8 and Example 2-12 having a totalcontent of the fibrillated heat-resistant fibers (I) and the fibrillatedheat-resistant fibers (II) of not less than 2.0 mass % to less than 9.0mass % based on the total of all the fiber components contained in thesubstrates were excellent in strength characteristic. The separatorshaving the respective substrates of Examples 2-1 to 2-6 and Example 2-12having a total content of the fibrillated heat-resistant fibers (I) andthe fibrillated heat-resistant fibers (II) of not less than 2.0 mass %to less than 9.0 mass %, a content of the fibrillated heat-resistantfibers (I) of not less than 1.0 mass % to less than 5.0 mass % and acontent of the fibrillated heat-resistant fibers (II) of not less than1.0 mass % to less than 4.0 mass % and the inorganic particle layer hadhigh adhesion between the substrate and the inorganic particle layer andexcellent cuttability. Further, they had low internal resistance, littleleakage current and excellent strength characteristic though thesubstrates had a small thickness of not more than 15 μm.

Out of the substrates manufactured in Examples 2-1 to 2-12, thesubstrate of Example 2-7 contained the fibrillated heat-resistant fibers(I), the fibrillated heat-resistant fibers (II) and the synthetic resinshort fibers and had a total content of the fibrillated heat-resistantfibers (I) and the fibrillated heat-resistant fibers (II) of not lessthan 2.0 mass % to less than 9.0 mass % and a content of the fibrillatedheat-resistant fibers (I) of not less than 5.0 mass % based on the totalof all the fiber components contained in the substrate. Therefore, asthe coating solution hardly permeated the substrate, adhesion betweenthe substrate and the inorganic particle layer lowered. In theevaluation of cuttability, fluff slightly remained on the edge surfaces.Further, it was found that the linear pressure had to be raised at thetime of controlling the thickness of the substrate to a predeterminedvalue by heat calender, thereby making it difficult to reduce thethickness of the substrate. As a result, the internal resistance of thebattery deteriorated.

Out of the substrates manufactured in Examples 2-1 to 2-12, thesubstrate of Example 2-8 contained the fibrillated heat-resistant fibers(I), the fibrillated heat-resistant fibers (II) and the synthetic resinshort fibers and had a total content of the fibrillated heat-resistantfibers (I) and the fibrillated heat-resistant fibers (II) of not lessthan 2.0 mass % to less than 9.0 mass % and a content of the fibrillatedheat-resistant fibers (II) of not less than 4.0 mass % based on thetotal of all the fiber components contained in the substrate. Fromcomparison between Example 2-5 and Example 2-8, when the fibrillatedheat-resistant fibers (II) were contained in an amount of not less than4.0 mass %, a large number of thick stem fibers remained, whereby thecoating solution hardly permeated the substrate, thereby reducingadhesion between the substrate and the inorganic particle layer. In theevaluation of cuttability, a large amount of fluff remained on the edgesurfaces. Further, it was found that the linear pressure had to beraised at the time of controlling the thickness of the substrate to apredetermined value by heat calender, thereby making it difficult toreduce the thickness of the substrate. As a result, the internalresistance of the battery deteriorated.

Out of the substrates manufactured in Examples 2-1 to 2-12, in Example2-9, the total content of the fibrillated heat-resistant fibers (I) andthe fibrillated heat-resistant fibers (II) was not less than 9.0 mass %,the content of the fibrillated heat-resistant fibers (I) was not lessthan 5.0 mass %, and the content of the fibrillated heat-resistantfibers (II) was not less than 4.0 mass % based on the total of all thefiber components contained in the substrate. In this case, the coatingsolution rarely permeated the substrate, whereby adhesion between thesubstrate and the inorganic particle layer lowered. In the evaluation ofcuttability, a large amount of fluff remained on the edge surfaces.Further, it was found that the linear pressure had to be raised to ahigher value than those of Examples 2-7 and Example 2-8 in which thetotal content of the fibrillated heat-resistant fibers (I) and thefibrillated heat-resistant fibers (II) was not less than 2.0 mass % toless than 9.0 mass % at the time of controlling the thickness of thesubstrate to a predetermined value by heat calender, thereby making itdifficult to reduce the thickness of the substrate. As a result, theinternal resistance of the battery deteriorated. Further, tensilestrength lowered.

Out of the substrates manufactured in Examples 2-1 to 2-12, thesubstrates of Examples 2-10 and 2-11 contained fibers other than thefibrillated heat-resistant fibers and the synthetic resin short fibers.From comparison between Example 2-10 and Example 2-11, the substrate ofExample 2-10 having a content of the synthetic resin short fibers of notless than 90.0 mass % was superior in strength characteristic to thesubstrate of Example 2-11 having a content of the synthetic resin shortfibers of less than 90.0 mass %.

Since the substrates of Comparative Examples 2-1 and 2-2 did not containheat-resistant fibers, when the basis weight of the substrate wasreduced, a leakage current was apt to be produced.

The substrate of Comparative Example 2-3 contained only the fibrillatedheat-resistant fibers (II) as the heat-resistant fibers, the substrateof Comparative Example 2-4 contained only the fibrillated heat-resistantfibers (I) as the heat-resistant fibers, and both of the substrates hada content of the heat-resistant fibers of 8.9 mass %. Although thecontent of the fibrillated heat-resistant fibers was not less than 2.0mass % to less than 9.0 mass %, when the substrate contained only one ofthese fibrillated heat-resistant fibers, adhesion between the substrateand the inorganic particle layer lowered (Comparative Example 2-3) andfluff remained on the edge surface in the evaluation of cuttability(Comparative Examples 2-3 and 2-4). Further, it was found that thelinear pressure had to be raised at the time of controlling thethickness of the substrate to a predetermined value by heat calender,thereby making it difficult to reduce the thickness of the substrate(Comparative Examples 2-3 and 2-4). As a result, the internal resistanceof the battery further deteriorated (Comparative Examples 2-3 and 2-4).

Example 3 Example 3-1

<Manufacture of Substrate>

55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.),1.0 part by mass of fibrid made of para-aromatic polyamide having amodified freeness of 88 ml obtained by defibrating and beating with ahigh-speed homogenizer and 3.5 parts by mass of fibrillatedheat-resistant fibers obtained by fibrillating a pulp-like product ofwholly aromatic polyamide fibers to a modified freeness of up to 50 mlwith a high-pressure homogenizer as heat-resistant fibers were dispersedin water with a pulper to prepare homogeneous papermaking slurry havinga concentration of 0.1 mass %, and a wet paper web was obtained by usingan inclination type paper machine and dried with a cylinder drier havinga surface temperature of 135° C. to obtain a sheet. The obtained sheetwas thermally calendered with a heat calender having a steel roll platedwith chromium and having a surface temperature of 195° C. as one of therolls and a resin roll having a hardness of Shore A92 as the other rolland a linear pressure of 100 kN/m to produce a substrate 3-1 having abasis weight of 10 g/m² and a thickness of 15 μm.

<Preparation of Coating Solution>

100 parts of boehmite having a particle diameter of 0.9 μm and aspecific surface area of 5.5 m²/g was mixed with 120 parts of a 0.3%aqueous solution of a carboxymethyl cellulose sodium salt having aviscosity at 25° C. of a 1 mass % aqueous solution thereof of 200 mPa·sand fully stirred, and then the resulting solution was mixed with 300parts of a 0.5% aqueous solution of a carboxymethyl cellulose sodiumsalt having a viscosity at 25° C. of a 1 mass % aqueous solution thereofof 7,000 mPa·s and 10 parts of a styrene butadiene rubber (SBR)-basedbinder for lithium ion batteries (TRD2001 of JSR Corporation) (solidconcentration of 48%) and stirred to prepare a coating solution.

<Manufacture of Separator>

The coating solution was applied to one side of the above substrate 3-1with a kiss reverse gravure coater to a coating weight of 12 g/m² anddried to manufacture a separator having an inorganic particle layer.

Example 3-2

A substrate 3-2 having a basis weight of 10 g/m² and a thickness of 15μm was manufactured in the same manner as the substrate 3-1 except that55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.0 parts by mass of the fibrid used in Example 3-1 and2.5 parts by mass of the fibrillated heat-resistant fibers used inExample 3-1 were used. Then, the coating solution prepared in Example3-1 was applied to a coating weight of 12 g/m² in the same manner as inExample 3-1 and dried to manufacture a separator.

Example 3-3

A substrate 3-3 having a basis weight of 10 g/m² and a thickness of 15μm was manufactured in the same manner as the substrate 3-1 except that55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.5 parts by mass of the fibrid used in Example 3-1 and2.0 parts by mass of the fibrillated heat-resistant fibers used inExample 3-1 were used. Then, the coating solution prepared in Example3-1 was applied to a coating weight of 12 g/m² in the same manner as inExample 3-1 and dried to manufacture a separator.

Example 3-4

A substrate 3-4 having a basis weight of 10 g/m² and a thickness of 15μm was manufactured in the same manner as the substrate 3-1 except that55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 3.5 parts by mass of the fibrid used in Example 3-1 and1.0 parts by mass of the fibrillated heat-resistant fibers used inExample 3-1 were used. Then, the coating solution prepared in Example3-1 was applied to a coating weight of 12 g/m² in the same manner as inExample 3-1 and dried to manufacture a separator.

Example 3-5

A substrate 3-5 having a basis weight of 10 g/m² and a thickness of 15μm was manufactured in the same manner as the substrate 3-1 except that59.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 0.4 part by mass of the fibrid used in Example 3-1 and0.6 parts by mass of the fibrillated heat-resistant fibers used inExample 3-1 were used. Then, the coating solution prepared in Example3-1 was applied to a coating weight of 12 g/m² in the same manner as inExample 3-1 and dried to manufacture a separator.

Example 3-6

A substrate 3-6 having a basis weight of 8 g/m² and a thickness of 12 μmwas manufactured in the same manner as the substrate 3-1 except that56.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 1.0 part by mass of the fibrid used in Example 3-1 and2.5 parts by mass of the fibrillated heat-resistant fibers used inExample 3-1 were used. Then, the coating solution prepared in Example3-1 was applied to a coating weight of 12 g/m² in the same manner as inExample 3-1 and dried to manufacture a separator.

Example 3-7

A substrate 3-7 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 3-1 except that55.1 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 1.0 part by mass of the fibrid used in Example 3-1 and3.9 parts by mass of the fibrillated heat-resistant fibers used inExample 3-1 were used. Then, the coating solution prepared in Example3-1 was applied to a coating weight of 12 g/m² in the same manner as inExample 3-1 and dried to manufacture a separator.

Example 3-8

A substrate 3-8 having a basis weight of 8 g/m² and a thickness of 12 μmwas manufactured in the same manner as the substrate 3-1 except that50.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.0 parts by mass of the fibrid used in Example 3-1, 2.5parts by mass of the fibrillated heat-resistant fibers used in Example3-1 and 5.5 parts by mass of solvent spun cellulose fibers obtained byrefining solvent spun cellulose fibers having an average fiber diameterof 10 μm and a fiber length of 4 mm with a refiner and beating to amodified freeness of 90 ml were used. Then, the coating solutionprepared in Example 3-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 3-1 and dried to manufacture a separator.

Example 3-9

A substrate 3-9 having a basis weight of 8 g/m² and a thickness of 12 μmwas manufactured in the same manner as the substrate 3-1 except that49.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.0 parts by mass of the fibrid used in Example 3-1, 2.5parts by mass of the fibrillated heat-resistant fibers used in Example3-1 and 6.5 parts by mass of the solvent spun cellulose fibers used inExample 3-8 were used. Then, the coating solution prepared in Example3-1 was applied to a coating weight of 12 g/m² in the same manner as inExample 3-1 and dried to manufacture a separator.

Comparative Example 3-1

A substrate 3-10 having a basis weight of 10 g/m² and a thickness of 15μm was manufactured in the same manner as the substrate 3-1 except that60.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mmand 40.0 parts by mass of undrawn PET-based synthetic resin short fibersfor single-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)were used. Then, the coating solution prepared in Example 3-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example3-1 and dried to manufacture a separator.

Comparative Example 3-2

A substrate 3-11 having a basis weight of 8 g/m² and a thickness of 12μm was manufactured in the same manner as the substrate 3-1 except that60.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mmand 40.0 parts by mass of undrawn PET-based synthetic resin short fibersfor single-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)were used. Then, the coating solution prepared in Example 3-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example3-1 and dried to manufacture a separator.

Comparative Example 3-3

A substrate 3-12 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 3-1 except that55.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.5 parts by mass of the fibrid used in Example 3-1 and2.5 parts by mass of the fibrillated heat-resistant fibers used inExample 3-1 were used. Then, the coating solution prepared in Example3-1 was applied to a coating weight of 12 g/m² in the same manner as inExample 3-1 and dried to manufacture a separator.

Comparative Example 3-4

A substrate 3-13 having a basis weight of 8 g/m² and a thickness of 12μm was manufactured in the same manner as the substrate 3-1 except that55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.),2.0 parts by mass of fibrid made of para-aromatic polyamide having amodified freeness of 310 ml and 2.5 parts by mass of fibrillatedheat-resistant fibers having a modified freeness of 320 ml were used andheat calender was carried out at a steel roll surface temperature of195° C. and a linear pressure of 120 kN/m to make the substrate uniformin thickness. Then, the coating solution prepared in Example 3-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example3-1 and dried to manufacture a separator.

TABLE 3-1 Ex. 3-1 Ex. 3-2 Ex. 3-3 Ex. 3-4 Ex. 3-5 Sub. 3-1 Sub. 3-2 Sub.3-3 Sub. 3-4 Sub. 3-5 Fiber oriented crystallized pbm 55.5 55.5 55.555.5 59.0 blending PET-based synthetic resin short fibers 0.06 dtex × 3mm Undrawn PET-based synthetic pbm 40.0 40.0 40.0 40.0 40.0 resin shortfibers 0.2 dtex × 3 mm Fibrillated heat-resistant pbm 3.5 2.5 2.0 1.00.6 fibers Fibrid pbm 1.0 2.0 2.5 3.5 0.4 Beaten melt spun cellulose pbm0.0 0.0 0.0 0.0 0.0 fibers Content of heat-resistant mass % 4.5 4.5 4.54.5 1.0 fibers having a modified freeness of not more than 300 mlFibrillated Modified freeness ml 50 50 50 50 50 heat-resistantMass-weighted average fiber mm 0.58 0.58 0.58 0.58 0.58 fibers lengthLength-weighted average mm 0.33 0.33 0.33 0.33 0.33 fiber length Fiberwidth μm 15.5 15.5 15.5 15.5 15.5 Fibrid Modified freeness ml 88 88 8888 88 Mass-weighted average fiber mm 0.52 0.52 0.52 0.52 0.52 lengthLength-weighted average mm 0.32 0.32 0.32 0.32 0.32 fiber length Fiberwidth μm 24.1 24.1 24.1 24.1 24.1 Linear pressure of heat calender kN/m100 100 100 100 100 Physical Basis weight of substrate g/m² 10 10 10 1010 properties Thickness of substrate μm 15 15 15 15 15 Coating weight ofcoating g/m² 12 12 12 12 12 layer Thickness of separator μm 25 25 25 2525 Evaluation Adhesion between substrate — ◯ ◯ ◯ ◯ ◯ and coating layerCuttability of separator — ◯ ◯ ◯ ◯ ◯ Internal resistance — ⊚ ⊚ ◯ Δ ⊚Leakage current — ⊚ ⊚ ⊚ ⊚ ⊚ Tensile strength — ⊚ ⊚ ⊚ ⊚ ⊚ Ex.: ExampleSub.: Substrate pbm: part by mass

TABLE 3-2 Ex. 3-6 Ex. 3-7 Ex. 3-8 Ex. 3-9 Sub. 3-6 Sub. 3-7 Sub. 3-8Sub. 3-9 Fiber oriented crystallized pbm 56.5 55.1 50.0 49.0 blendingPET-based synthetic resin short fibers 0.06 dtex × 3 mm UndrawnPET-based synthetic pbm 40.0 40.0 40.0 40.0 resin short fibers 0.2 dtex× 3 mm Fibrillated heat-resistant pbm 2.5 3.9 2.5 2.5 fibers Fibrid pbm1.0 1.0 2.0 2.0 Beaten melt spun cellulose pbm 0.0 0.0 5.5 6.5 fibersContent of heat-resistant mass % 3.5 4.9 4.5 4.5 fibers having amodified freeness of not more than 300 ml Fibrillated Modified freenessml 50 50 50 50 heat-resistant Mass-weighted average fiber mm 0.58 0.580.58 0.58 fibers length Length-weighted average mm 0.33 0.33 0.33 0.33fiber length Fiber width μm 15.5 15.5 15.5 15.5 Fibrid Modified freenessml 88 88 88 88 Mass-weighted average fiber mm 0.52 0.52 0.52 0.52 lengthLength-weighted average mm 0.32 0.32 0.32 0.32 fiber length Fiber widthμm 24.1 24.1 24.1 24.1 Linear pressure of heat calender kN/m 100 100 100100 Physical Basis weight of substrate g/m² 8 6 8 8 properties Thicknessof substrate μm 12 9 12 12 Coating weight of coating g/m² 12 12 12 12layer Thickness of separator μm 22 19 22 22 Evaluation Adhesion betweensubstrate — ◯ ◯ ◯ ◯ and coating layer Cuttability of separator — ◯ ◯ ◯ ◯Internal resistance — ⊚ ⊚ ⊚ ⊚ Leakage current — ⊚ ⊚ ⊚ ⊚ Tensile strength— ◯ ◯ ◯ Δ Ex.: Example Sub.: Substrate pbm: part by mass

TABLE 3-3 C. Ex. 3-1 C. Ex. 3-2 C. Ex. 3-3 C. Ex. 3-4 Sub. 3-10 Sub.3-11 Sub. 3-12 Sub. 3-13 Fiber oriented crystallized pbm 60.0 60.0 55.055.5 blending PET-based synthetic resin short fibers 0.06 dtex × 3 mmUndrawn PET-based synthetic pbm 40.0 40.0 40.0 40.0 resin short fibers0.2 dtex × 3 mm Fibrillated heat-resistant pbm 0.0 0.0 2.5 2.5 fibersFibrid pbm 0.0 0.0 2.5 2.0 Beaten melt spun cellulose pbm 0 0 0 0 fibersContent of heat-resistant mass % 0.0 0.0 5.0 0.0 fibers having amodified freeness of not more than 300 ml Fibrillated Modified freenessml — — 50 320 heat-resistant Mass-weighted average fiber mm — — 0.581.30 fibers length Length-weighted average fiber mm — — 0.33 0.59 lengthFiber width μm — — 15.5 25.2 Fibrid Modified freeness ml — — 88 310Mass-weighted average fiber mm — — 0.52 0.78 length Length-weightedaverage mm — — 0.32 0.56 fiber length Fiber width μm — — 24.1 31.2Linear pressure of heat calender kN/m 100 100 100 120 Physical Basisweight of substrate g/m² 10 8 6 8 properties Thickness of substrate μm15 12 9 12 Coating weight of coating g/m² 12 12 12 12 layer Thickness ofseparator μm 25 22 19 22 Evaluation Adhesion between substrate — ◯ ◯ Δ Δand coating layer Cuttability of separator — ◯ ◯ ◯ Δ Internal resistance— ◯ ⊚ Δ Δ Leakage current — Δ X ◯ Δ Tensile strength — ⊚ ◯ ◯ Δ C. Ex.:Comparative Example Sub.: Substrate pbm: part by mass

The substrates manufactured in Examples 3-1 to 3-9 which containedheat-resistant fibers and synthetic resin short fibers contained thefibrillated heat-resistant fibers (I) and the fibrid as theheat-resistant fibers and had a total content of the fibrillatedheat-resistant fibers (I) and the fibrid of not less than 1.0 mass % toless than 5.0 mass based on the total of all the fiber componentscontained in the substrate. The separators having the respectivesubstrates of Examples 3-1 to 3-9 and the inorganic particle layer hadhigh adhesion between the substrate and the coating layer and excellentcuttability. They had low internal resistance and little leakagecurrent. Although the substrates were as thin as not more than 15 μm,they had excellent strength characteristic. Especially the substrates ofExamples 3-6 and 3-7 had a content of the fibrillated heat-resistantfibers (I) of 71 mass % and 80 mass % based on the total of thefibrillated heat-resistant fibers (I) and the fibrid, respectively, thenumber of fiber networks increased, and the fibrid firmly bonded thefiber networks together. Although the basis weights of the substrateswere only 8 g/m² and 6 g/m², respectively, the evaluation results ofinternal resistance and leakage current were satisfactory, and strengthcharacteristic could be maintained.

It was found from comparison between Examples 3-1 to 3-2 and Examples3-3 to 3-4 that, when the basis weight of the substrate was 10 g/m², inExamples 3-1 and 3-2 in which the ratio of the content of thefibrillated heat-resistant fibers (I) to the total content of thefibrillated heat-resistant fibers (I) and the fibrid was higher,internal resistance was lower and satisfactory.

The substrates of Examples 3-8 and 3-9 contained fibers other than thefibrillated heat-resistant fibers (I), the fibrid and the syntheticresin short fibers. From comparison between Examples 3-8 and Example3-9, the substrate of Example 3-8 in which the content of the syntheticresin short fibers was not less than 90.0 mass % was superior instrength characteristic.

Since the substrates of Comparative Examples 3-1 and 3-2 did not containheat-resistant fibers, when the basis weight of the substrate wasreduced, a leakage current was apt to be produced.

Although the substrate of Comparative Example 3-3 had a basis weight of6 g/m² and a total content of the fibrillated heat-resistant fibers (I)and the fibrid of more than 5 mass %, adhesion between the substrate andthe coating layer, internal resistance and strength characteristicdeteriorated.

Since the modified freeness's of the fibrillated heat-resistant fibersand fibrid of the substrate of Comparative Example 3-4 exceeded 300 ml,the fiber width of the fibrid and the fiber diameter of the stem part ofeach of the fibrillated heat-resistant fibers were large, whereby thedispersibility of the fibers and entanglement between the fibers becameworse, thereby deteriorating the texture of the substrate. As a result,adhesion between the substrate and the coating layer and thecuttability, internal resistance and leakage current of the separatordeteriorated. The strength characteristic of the substrate also becameworse.

Example 4 Example 4-1

<Manufacture of Substrate>

55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.),1.0 part by mass of fibrid made of para-aromatic polyamide having amodified freeness of 88 ml obtained by defibrating and beating with ahigh-speed homogenizer, 1.0 part by mass of fibrillated heat-resistantfibers (II) obtained by fibrillating a pulp-like product (average fiberlength of 1.7 mm, average fiber diameter of 10 μm) of para-aromaticpolyamide fibers to a modified freeness of up to 350 ml with ahigh-pressure homogenizer and 2.5 parts by mass of fibrillatedheat-resistant fibers (I) obtained by fibrillating a pulp-like product(average fiber length of 1.7 mm, average fiber diameter of 10 μm) ofpara-aromatic polyamide fibers to a modified freeness of up to 50 mlwith a high-pressure homogenizer as heat-resistant fibers were dispersedin water with a pulper to prepare homogeneous papermaking slurry havinga concentration of 0.1 mass %, and a wet paper web was obtained by usingan inclination type paper machine and dried with a cylinder drier havinga surface temperature of 135° C. to obtain a sheet. The obtained sheetwas calendered with a heat calender having a steel roll plated withchromium and having a surface temperature of 195° C. as one of the rollsand a resin roll having a hardness of Shore A92 as the other roll and alinear pressure of 100 kN/m to produce a substrate 4-1 having a basisweight of 10 g/m² and a thickness of 15 μm.

<Preparation of Coating Solution>

100 parts of boehmite having a D50 particle diameter of 0.9 μm and aspecific surface area of 5.5 m²/g was mixed with 120 parts of a 0.3%aqueous solution of a carboxymethyl cellulose sodium salt having aviscosity at 25° C. of a 1 mass % aqueous solution thereof of 200 mPa·sand fully stirred, and then the resulting solution was mixed with 300parts of a 0.5% aqueous solution of a carboxymethyl cellulose sodiumsalt having a viscosity at 25° C. of a 1 mass % aqueous solution thereofof 7,000 mPa·s and 10 parts of a styrene butadiene rubber (SBR)-basedbinder for lithium ion batteries (TRD2001 of JSR Corporation) (solidconcentration of 48%) and stirred to prepare a coating solution.

<Manufacture of Separator>

The coating solution was applied to one side of the above substrate 4-1with a kiss reverse gravure coater to a coating weight of 12 g/m² anddried to manufacture a separator having an inorganic particle layer.

Example 4-2

A substrate 4-2 having a basis weight of 10 g/m² and a thickness of 15μm was manufactured in the same manner as the substrate 4-1 except that55.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 0.4 part by mass of the fibrid used in Example 4-1, 3.5parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1 and 0.6 part by mass of the fibrillated heat-resistantfibers (I) used in Example 4-1 were used. Then, the coating solutionprepared in Example 4-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 4-1 and dried to manufacture a separator.

Example 4-3

A substrate 4-3 having a basis weight of 10 g/m² and a thickness of 15μm was manufactured in the same manner as the substrate 4-1 except that58.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 0.4 part by mass of the fibrid used in Example 4-1, 1.0part by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1 and 0.6 part by mass of the fibrillated heat-resistantfibers (I) used in Example 4-1 were used. Then, the coating solutionprepared in Example 4-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 4-1 and dried to manufacture a separator.

Example 4-4

A substrate 4-4 having a basis weight of 8 g/m² and a thickness of 12 μmwas manufactured in the same manner as the substrate 4-1 except that55.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 1.0 part by mass of the fibrid used in Example 4-1, 1.5parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1 and 2.5 parts by mass of the fibrillated heat-resistantfibers (I) used in Example 4-1 were used. Then, the coating solutionprepared in Example 4-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 4-1 and dried to manufacture a separator.

Example 4-5

A substrate 4-5 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that51.6 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.0 parts by mass of the fibrid used in Example 4-1, 3.9parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1 and 2.5 parts by mass of the fibrillated heat-resistantfibers (I) used in Example 4-1 were used. Then, the coating solutionprepared in Example 4-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 4-1 and dried to manufacture a separator.

Example 4-6

A substrate 4-6 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that52.6 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 0.4 part by mass of the fibrid used in Example 4-1, 2.5parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1 and 4.5 parts by mass of the fibrillated heat-resistantfibers (I) used in Example 4-1 were used. Then, the coating solutionprepared in Example 4-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 4-1 and dried to manufacture a separator.

Example 4-7

A substrate 4-7 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that52.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.5 parts by mass of the fibrid used in Example 4-1, 2.5parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1 and 2.5 parts by mass of the fibrillated heat-resistantfibers (I) used in Example 4-1 were used. Then, the coating solutionprepared in Example 4-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 4-1 and dried to manufacture a separator.

Example 4-8

A substrate 4-8 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that51.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.0 parts by mass of the fibrid used in Example 4-1, 4.0parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1 and 2.5 parts by mass of the fibrillated heat-resistantfibers (I) used in Example 4-1 were used and heat calender was carriedout at a steel roll surface temperature of 195° C. and a linear pressureof 110 kN/m to make the substrate uniform in thickness. Then, thecoating solution prepared in Example 4-1 was applied to a coating weightof 12 g/m² in the same manner as in Example 4-1 and dried to manufacturea separator.

Example 4-9

A substrate 4-9 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that52.5 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 0.4 part by mass of the fibrid used in Example 4-1, 2.5parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1 and 4.6 parts by mass of the fibrillated heat-resistantfibers (I) used in Example 4-1 were used. Then, the coating solutionprepared in Example 4-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 4-1 and dried to manufacture a separator.

Example 4-10

A substrate 4-10 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that51.4 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.5 parts by mass of the fibrid used in Example 4-1, 1.5parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1 and 4.6 parts by mass of the fibrillated heat-resistantfibers (I) used in Example 4-1 were used and heat calender was carriedout at a steel roll surface temperature of 195° C. and a linear pressureof 110 kN/m to make the substrate uniform in thickness. Then, thecoating solution prepared in Example 4-1 was applied to a coating weightof 12 g/m² in the same manner as in Example 4-1 and dried to manufacturea separator.

Example 4-11

A substrate 4-11 having a basis weight of 7 g/m² and a thickness of 11μm was manufactured in the same manner as the substrate 4-1 except that50.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.0 parts by mass of the fibrid used in Example 4-1, 2.5parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1, 2.5 parts by mass of the fibrillated heat-resistant fibers(I) used in Example 4-1 and 3.0 parts by mass of solvent spun cellulosefibers obtained by refining solvent spun cellulose fibers having anaverage fiber diameter of 10 μm and a fiber length of 4 mm with arefiner and beating to a modified freeness of 90 ml were used. Then, thecoating solution prepared in Example 4-1 was applied to a coating weightof 12 g/m² in the same manner as in Example 4-1 and dried to manufacturea separator.

Example 4-12

A substrate 4-12 having a basis weight of 7 g/m² and a thickness of 11μm was manufactured in the same manner as the substrate 4-1 except that49.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.0 parts by mass of the fibrid used in Example 4-1, 2.5parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1, 2.5 parts by mass of the fibrillated heat-resistant fibers(I) used in Example 4-1 and 4.0 parts by mass of the solvent spuncellulose fibers used in Example 4-11 were used. Then, the coatingsolution prepared in Example 4-1 was applied to a coating weight of 12g/m² in the same manner as in Example 4-1 and dried to manufacture aseparator.

Comparative Example 4-1

A substrate 4-13 having a basis weight of 8 g/m² and a thickness of 12μm was manufactured in the same manner as the substrate 4-1 except that60.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mmand 40.0 parts by mass of undrawn PET-based synthetic resin short fibersfor single-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)were used. Then, the coating solution prepared in Example 4-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example4-1 and dried to manufacture a separator.

Comparative Example 4-2

A substrate 4-14 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that60.0 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mmand 40.0 parts by mass of undrawn PET-based synthetic resin short fibersfor single-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)were used. Then, the coating solution prepared in Example 4-1 wasapplied to a coating weight of 12 g/m² in the same manner as in Example4-1 and dried to manufacture a separator.

Comparative Example 4-3

A substrate 4-15 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that50.9 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm, 2.4 parts by mass of the fibrid used in Example 4-1, 3.9parts by mass of the fibrillated heat-resistant fibers (II) used inExample 4-1 and 2.8 parts by mass of the fibrillated heat-resistantfibers (I) used in Example 4-1 were used and heat calender was carriedout at a steel roll surface temperature of 195° C. and a linear pressureof 120 kN/m to make the substrate uniform in thickness. Then, thecoating solution prepared in Example 4-1 was applied to a coating weightof 12 g/m² in the same manner as in Example 4-1 and dried to manufacturea separator.

Comparative Example 4-4

A substrate 4-16 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that51.1 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)and 8.9 parts by mass of the fibrid used in Example 4-1 were used. Then,the coating solution prepared in Example 4-1 was applied to a coatingweight of 12 g/m² in the same manner as in Example 4-1 and dried tomanufacture a separator.

Comparative Example 4-5

A substrate 4-17 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that51.1 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)and 8.9 parts by mass of the fibrillated heat-resistant fibers (II) usedin Example 4-1 were used and heat calender was carried out at a steelroll surface temperature of 195° C. and a linear pressure of 120 kN/m tomake the substrate uniform in thickness. Then, the coating solutionprepared in Example 4-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 4-1 and dried to manufacture a separator.

Comparative Example 4-6

A substrate 4-18 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that51.1 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.)and 8.9 parts by mass of the fibrillated heat-resistant fibers (I) usedin Example 4-1 were used and heat calender was carried out at a steelroll surface temperature of 195° C. and a linear pressure of 120 kN/m tomake the substrate uniform in thickness. Then, the coating solutionprepared in Example 4-1 was applied to a coating weight of 12 g/m² inthe same manner as in Example 4-1 and dried to manufacture a separator.

Comparative Example 4-7

A substrate 4-19 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that51.1 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.),3.9 parts by mass of the fibrid used in Example 4-1 and 5.0 parts bymass of the fibrillated heat-resistant fibers (I) used in Example 4-1were used and heat calender was carried out at a steel roll surfacetemperature of 195° C. and a linear pressure of 120 kN/m to make thesubstrate uniform in thickness. Then, the coating solution prepared inExample 4-1 was applied to a coating weight of 12 g/m² in the samemanner as in Example 4-1 and dried to manufacture a separator.

Comparative Example 4-8

A substrate 4-20 having a basis weight of 6 g/m² and a thickness of 9 μmwas manufactured in the same manner as the substrate 4-1 except that51.1 parts by mass of oriented crystallized PET-based synthetic resinshort fibers having a fineness of 0.06 dtex and a fiber length of 3 mm,40.0 parts by mass of undrawn PET-based synthetic resin short fibers forsingle-component binders having a fineness of 0.2 dtex and a fiberlength of 3 mm (softening point of 120° C., melting point of 230° C.),3.9 parts by mass of the fibrid used in Example 4-1 and 5.0 parts bymass of the fibrillated heat-resistant fibers (II) used in Example 4-1were used and heat calender was carried out at a steel roll surfacetemperature of 195° C. and a linear pressure of 120 kN/m to make thesubstrate uniform in thickness. Then, the coating solution prepared inExample 4-1 was applied to a coating weight of 12 g/m² in the samemanner as in Example 4-1 and dried to manufacture a separator.

TABLE 4-1 Ex. 4-1 Ex. 4-2 Ex. 4-3 Ex. 4-4 Ex. 4-5 Ex. 4-6 Sub. 4-1 Sub.4-2 Sub. 4-3 Sub. 4-4 Sub. 4-5 Sub. 4-6 Fiber oriented crystallized pbm55.5 55.5 58.0 55.0 51.6 52.6 blending PET-based synthetic resin shortfibers 0.06 dtex × 3 mm Undrawn PET-based synthetic pbm 40.0 40.0 40.040.0 40.0 40.0 resin short fibers 0.2 dtex × 3 mm Fibrillatedheat-resistant pbm 2.5 0.6 0.6 2.5 2.5 4.5 fibers(I) Fibrillatedheat-resistant pbm 1.0 3.5 1.0 1.5 3.9 2.5 fibers(II) Fibrid pbm 1.0 0.40.4 1.0 2.0 0.4 heat-resistant fiber pbm 4.5 4.5 2.0 5.0 8.4 7.4 Beatenmelt spun cellulose pbm 0.0 0.0 0.0 0.0 0.0 0.0 fibers FibrillatedModified freeness ml 50 50 50 50 50 50 heat-resistant Mass-weightedaverage fiber mm 0.58 0.58 0.58 0.58 0.58 0.58 fibers (I) lengthLength-weighted average mm 0.33 0.33 0.33 0.33 0.33 0.33 fiber lengthFiber width μm 15.5 15.5 15.5 15.5 15.5 15.5 Fibrillated Modifiedfreeness ml 350 350 350 350 350 350 heat-resistant Mass-weighted averagefiber mm 1.30 1.30 1.30 1.30 1.30 1.30 fibers (II) lengthLength-weighted average mm 0.59 0.59 0.59 0.59 0.59 0.59 fiber lengthFiber width μm 25.2 25.2 25.2 25.2 25.2 25.2 Fibrid Modified freeness ml88 88 88 88 88 88 Mass-weighted average fiber mm 0.52 0.52 0.52 0.520.52 0.52 length Length-weighted average mm 0.32 0.32 0.32 0.32 0.320.32 fiber length Fiber width μm 24.1 24.1 24.1 24.1 24.1 24.1 Linearpressure of heat calender kN/m 100 100 100 100 100 100 Physical Basisweight of substrate g/m² 10 10 10 8 6 6 properties Thickness ofsubstrate μm 15 15 15 12 9 9 Coating weight of coating g/m² 12 12 12 1212 12 layer Thickness of separator μm 25 25 25 22 19 19 EvaluationAdhesion between substrate — ◯ ◯ ◯ ◯ ◯ ◯ and coating layer Cuttabilityof separator — ◯ ◯ ◯ ◯ ◯ ◯ Internal resistance — ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Leakagecurrent — ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Tensile strength — ⊚ ⊚ ⊚ ◯ ◯ ◯ Ex.: Example Sub.:Substrate pbm: part by mass

TABLE 4-2 Ex. 4-7 Ex. 4-8 Ex. 4-9 Ex. 4-10 Ex. 4-11 Ex. 4-12 Sub. 4-7Sub. 4-8 Sub. 4-9 Sub. 4-10 Sub. 4-11 Sub. 4-12 Fiber blending orientedcrystallized pbm 52.5 51.5 52.5 51.4 50.0 49.0 PET-based synthetic resinshort fibers 0.06 dtex × 3 mm Undrawn PET-based synthetic pbm 40.0 40.040.0 40.0 40.0 40.0 resin short fibers 0.2 dtex × 3 mm Fibrillatedheat-resistant pbm 2.5 2.5 4.6 4.6 2.5 2.5 fibers (I) Fibrillatedheat-resistant pbm 2.5 4.0 2.5 1.5 2.5 2.5 fibers (II) Fibrid pbm 2.52.0 0.4 2.5 2.0 2.0 heat-resistant fiber pbm 7.5 8.5 7.5 8.6 7.0 7.0Beaten melt spun cellulose pbm 0.0 0.0 0.0 0.0 3.0 4.0 fibersFibrillated Modified freeness ml 50 50 50 50 50 50 heat-resistantMass-weighted average fiber mm 0.58 0.58 0.58 0.58 0.58 0.58 fibers (I)length Length-weighted average mm 0.33 0.33 0.33 0.33 0.33 0.33 fiberlength Fiber width μm 15.5 15.5 15.5 15.5 15.5 15.5 Fibrillated Modifiedfreeness ml 350 350 350 350 350 350 heat-resistant Mass-weighted averagefiber mm 1.30 1.30 1.30 1.30 1.30 1.30 fibers (II) lengthLength-weighted average mm 0.59 0.59 0.59 0.59 0.59 0.59 fiber lengthFiber width μm 25.2 25.2 25.2 25.2 25.2 25.2 Fibrid Modified freeness ml88 88 88 88 88 88 Mass-weighted average fiber mm 0.52 0.52 0.52 0.520.52 0.52 length Length-weighted average mm 0.32 0.32 0.32 0.32 0.320.32 fiber length Fiber width μm 24.1 24.1 24.1 24.1 24.1 24.1 Linearpressure of heat calender kN/m 100 110 100 110 100 100 Physical Basisweight of substrate g/m² 6 6 6 6 7 7 properties Thickness of substrateμm 9 9 9 9 11 11 Coating weight of coating g/m² 12 12 12 12 12 12 layerThickness of separator μm 19 19 19 19 21 21 Evaluation Adhesion betweensubstrate — ◯ ◯ Δ Δ ◯ ◯ and coating layer Cuttability of separator — ◯ Δ◯ ◯ ◯ ◯ Internal resistance — Δ ◯ ◯ Δ ⊚ ⊚ Leakage current — ⊚ ⊚ ⊚ ⊚ ⊚ ⊚Tensile strength — ◯ ◯ ◯ ◯ ◯ Δ Ex.: Example Sub.: Substrate pbm: part bymass

TABLE 4-3 C. EX. 4-1 C. Ex. 4-2 C. Ex. 4-3 C. Ex. 4-4 Sub. 4-13 Sub.4-14 Sub. 4-15 Sub. 4-16 Fiber blending oriented crystallized pbm 60.060.0 50.9 51.1 PET-based synthetic resin short fibers 0.06 dtex × 3 mmUndrawn PET-based synthetic pbm 40.0 40.0 40.0 40.0 resin short fibers0.2 dtex × 3 mm Fibrillated heat-resistant pbm 0.0 0.0 2.8 0.0 fibers(I) Fibrillated heat-resistant pbm 0.0 0.0 3.9 0.0 fibers(II) Fibrid pbm0.0 0.0 2.4 8.9 heat-resistant fiber pbm 0.0 0.0 9.1 8.9 Beaten meltspun cellulose pbm 0.0 0.0 0.0 0.0 fibers Fibrillated Modified freenessml — — 50 — heat-resistant Mass-weighted average fiber mm — — 0.58 —fibers (I) length Length-weighted average mm — — 0.33 — fiber lengthFiber width μm — — 15.5 — Fibrillated Modified freeness ml — — 350 —heat-resistant Mass-weighted average fiber mm — — 1.30 — fibers (II)length Length-weighted average mm — — 0.59 — fiber length Fiber width μm— — 25.2 — Fibrid Modified freeness ml — — 88 88 Mass-weighted averagefiber mm — — 0.52 0.52 length Length-weighted average mm — — 0.32 0.32fiber length Fiber width μm — — 24.1 24.1 Linear pressure of heatcalender kN/m 100 100 120 100 Physical properties Basis weight ofsubstrate g/m² 8 6 6 6 Thickness of substrate μm 12 9 9 9 Coating weightof coating g/m² 12 12 12 12 layer Thickness of separator μm 21 19 19 19Evaluation Adhesion between substrate — ◯ ◯ X X and coating layerCuttability of separator — ◯ ◯ Δ Δ Internal resistance — ◯ ◯ X X Leakagecurrent — X X ◯ ⊚ Tensile strength — ◯ ◯ Δ ◯ C. EX. 4-5 C. Ex. 4-6 C.Ex. 4-7 C. Ex. 4-8 Sub. 4-17 Sub. 4-18 Sub. 4-19 Sub. 4-20 Fiberblending oriented crystallized pbm 51.1 51.1 51.1 51.1 PET-basedsynthetic resin short fibers 0.06 dtex × 3 mm Undrawn PET-basedsynthetic pbm 40.0 40.0 40.0 40.0 resin short fibers 0.2 dtex × 3 mmFibrillated heat-resistant pbm 0.0 8.9 5.0 0.0 fibers (I) Fibrillatedheat-resistant pbm 8.9 0.0 0.0 5.0 fibers(II) Fibrid pbm 0.0 0.0 3.9 3.9heat-resistant fiber pbm 8.9 8.9 8.9 8.9 Beaten melt spun cellulose pbm0.0 0.0 0.0 0.0 fibers Fibrillated Modified freeness ml — 50 50 —heat-resistant Mass-weighted average fiber mm — 0.58 0.58 — fibers (I)length Length-weighted average mm — 0.33 0.33 — fiber length Fiber widthμm — 15.5 15.5 — Fibrillated Modified freeness ml 350 — — 350heat-resistant Mass-weighted average fiber mm 1.30 — — 1.30 fibers (II)length Length-weighted average mm 0.59 — — 0.59 fiber length Fiber widthμm 25.2 — — 25.2 Fibrid Modified freeness ml — — 88 88 Mass-weightedaverage fiber mm — — 0.52 0.52 length Length-weighted average mm — —0.32 0.32 fiber length Fiber width μm — — 24.1 24.1 Linear pressure ofheat calender kN/m 120 120 120 120 Physical properties Basis weight ofsubstrate g/m² 6 6 6 6 Thickness of substrate μm 9 9 9 9 Coating weightof coating g/m² 12 12 12 12 layer Thickness of separator μm 19 19 19 19Evaluation Adhesion between substrate — Δ Δ X Δ and coating layerCuttability of separator — X X X Δ Internal resistance — X X X X Leakagecurrent — ◯ ⊚ ⊚ ◯ Tensile strength — Δ Δ Δ Δ C. Ex.: Comparative ExampleSub.: Substrate pbm: part by mass

The substrates manufactured in Examples 4-1 to 4-12 contained thefibrillated heat-resistant fibers (I), the fibrillated heat-resistantfibers (II), the fibrid and synthetic resin short fibers, and the totalcontent of the fibrillated heat-resistant fibers (I) and (II) and thefibrid was not less than 2.0 mass % to less than 9.0 mass % based on thetotal of all the fiber components contained in the substrates. Theseparators having the respective substrates of Examples 4-1 to 4-12 andthe inorganic particle layer had high adhesion between the substrate andthe inorganic particle layer and excellent cuttability. Further, theyhad low internal resistance, little leakage current and excellentstrength characteristic though the substrates were as thin as not morethan 15 μm. The separators having the respective substrates of Examples4-1 to 4-6 and Examples 4-11 to 4-12 in which the total content of thefibrillated heat-resistant fibers (I) and (II) and the fibrid was notless than 2.0 mass % to less than 9.0 mass %, the content of the fibridwas not less than 0.4 mass % to less than 2.5 mass %, the content of thefibrillated heat-resistant fibers (II) was not less than 1.0 mass % toless than 4.0 mass % and the content of the fibrillated heat-resistantfibers (I) was not less than 0.6 mass % to less than 4.6 mass % and theinorganic particle layer had high adhesion between the substrate and theinorganic particle layer and excellent cuttability. Further, they hadlow internal resistance, little leakage current and excellent strengthcharacteristic though the substrate was as thin as 15 μm.

Out of the substrates manufactured in Examples 4-1 to 4-12, thesubstrate of Example 4-7 had a total content of the fibrillatedheat-resistant fibers (I) and (II) and the fibrid of not less than 2.0mass % to less than 9.0 mass %, a content of the fibrillatedheat-resistant fibers (I) of not less than 0.6 mass % to less than 4.6mass % and a content of the fibrillated heat-resistant fibers (II) ofnot less than 1.0 mass % to less than 4.0 mass % but a content of thefibrid of not less than 2.5 mass %. Therefore, the separator includingthe substrate was excellent in leakage current but slightly deterioratedin internal resistance.

Out of the substrates manufactured in Examples 4-1 to 4-12, thesubstrate of Example 4-8 had a total content of the fibrillatedheat-resistant fibers (I) and (II) and the fibrid of not less than 2.0mass % to less than 9.0 mass %, a content of the fibrillatedheat-resistant fibers (I) of not less than 0.6 mass % to less than 4.6mass % and a content of the fibrid of not less than 0.4 mass % to lessthan 2.5 mass % but a content of the fibrillated heat-resistant fibers(II) of not less than 4.0 mass %. Therefore, as a large number of thickpara-aromatic polyamide stem fibers remained, fluff slightly remained onthe edge surfaces. Further, it was found that a high linear pressure hadto be applied at the time of controlling the thickness of the substrateto a predetermined value by heat calender, thereby making it difficultto reduce the thickness of the substrate. Since the basis weight was 6g/m², internal resistance was satisfactory.

Out of the substrates manufactured in Examples 4-1 to 4-12, thesubstrate of Example 4-9 had a total content of the fibrillatedheat-resistant fibers (I) and (II) and the fibrid of not less than 2.0mass % to less than 9.0 mass %, a content of the fibrid of not less than0.4 mass % to less than 2.5 mass % and a content of the fibrillatedheat-resistant fibers (II) of not less than 1.0 mass % to less than 4.0mass % but a content of the fibrillated heat-resistant fibers (I) of notless than 4.6 mass %. The total content of the fibrid and thefibrillated heat-resistant fibers (I) was not less than 5.0 mass %.Therefore, as the coating solution hardly permeated the substrate,adhesion between the substrate and the inorganic particle layer lowered.

Out of the substrates manufactured in Examples 4-1 to 4-12, thesubstrate of Example 10 had a total content of the fibrillatedheat-resistant fibers (I) and (II) and the fibrid of not less than 2.0mass % to less than 9.0 mass %, a content of the fibrillatedheat-resistant fibers (II) of not less than 1.0 mass % to less than 4.0mass % but a content of the fibrid of not less than 2.5 mass % and acontent of the fibrillated heat-resistant fibers (I) of not less than4.6 mass %. Therefore, as the coating solution hardly permeated thesubstrate, adhesion between the substrate and the inorganic particlelayer lowered. Further, since the total content of the fibrid and thefibrillated heat-resistant fibers (I) was not less than 5.0 mass %,internal resistance deteriorated though the basis weight was only 6g/m².

The substrates of Examples 4-11 and 4-12 contained fibers other than thefibrid, the fibrillated heat-resistant fibers (II), the fibrillatedheat-resistant fibers (I) and the synthetic resin short fibers. Fromcomparison between Example 4-11 and Example 4-12, the substrate ofExample 4-11 having a content of the synthetic resin short fibers of notless than 90.0 mass % was superior in strength characteristic.

Since the substrates of Comparative Examples 4-1 and 4-2 did not containthe fibrillated heat-resistant fibers (I) and (II) and the fibrid, whenthe basis weight of the substrate was reduced, a leakage current becameworse.

In Comparative Example 4-3, the content of the fibrillatedheat-resistant fibers (I) was not less than 0.6 mass % to less than 4.6mass %, the content of the fibrillated heat-resistant fibers (II) wasnot less than 1.0 mass % to less than 4.0 mass %, the content of thefibrid was not less than 0.4 mass % to less than 2.5 mass %, and thetotal content of the fibrillated heat-resistant fibers (I) and (II) andthe fibrid exceeded 9.0 mass %. Therefore, as the coating solutionhardly permeated the substrate, adhesion between the substrate and thecoating solution lowered. In the evaluation of cuttability, fluffslightly remained on the edge surfaces. Further, it was found that ahigh linear pressure had to be applied at the time of controlling thethickness of the substrate to a predetermined value by heat calender,thereby making it difficult to reduce the thickness of the substrate. Asa result, internal resistance deteriorated.

In Comparative Example 4-4, only the fibrid was contained in an amountof 8.9 mass %. Although the total content of the heat-resistant fiberswas not less than 2.0 mass % to less than 9.0 mass, when only the fibridwas contained, the coating solution hardly permeated the substrate,thereby reducing adhesion between the substrate and the coatingsolution. In the evaluation of cuttability, fluff slightly remained onthe edge surface. Since the fibrous form of the fibrid was thinleaf-like, internal resistance deteriorated.

In Comparative Example 4-5, only the fibrillated heat-resistant fibers(II) were contained in an amount of 8.9 mass %. Although the totalcontent of the heat-resistant fibers was not less than 2.0 mass % toless than 9.0 mass %, when only the fibrillated heat-resistant fibers(II) were contained, the coating solution hardly permeated thesubstrate, thereby reducing adhesion between the substrate and thecoating solution. Since a large number of thick stem fibers of thefibrillated heat-resistant fibers (II) remained, cutting was difficultand fluff remained on the edge surfaces. Further, it was found that ahigh linear pressure had to be applied at the time of controlling thethickness of the substrate to a predetermined value by heat calender,thereby making it difficult to reduce the thickness of the substrate. Asa result, internal resistance deteriorated. Since there are a largenumber of thick stem fibers of the fibrillated heat-resistant fibers(II), a fiber network became weak, thereby deteriorating strengthcharacteristic.

In Comparative Example 4-6, only the fibrillated heat-resistant fibers(I) were contained in an amount of 8.9 mass %. Although the totalcontent of the heat-resistant fibers was not less than 2.0 mass % toless than 9.0 mass %, when only the fibrillated heat-resistant fibers(I) were contained, the number of fine fibrillated fibers greatlyincreased, whereby the coating solution hardly permeated the substratewith the result that adhesion between the substrate and the coatingsolution lowered, cutting was difficult, and fluff remained on the edgesurfaces. Further, it was found that a high linear pressure had to beapplied at the time of controlling the thickness of the substrate to apredetermined value by heat calender, thereby making it difficult toreduce the thickness of the substrate. As a result, internal resistancedeteriorated. Since the number of fine fibril fibers increased too much,bonding between fibers became loose, thereby deteriorating strengthcharacteristic.

In Comparative Example 4-7, 5.0 mass % of the fibrillated heat-resistantfibers (I) and 3.9 mass % of the fibrid were contained. Although thetotal content of the heat-resistant fibers was not less than 2.0 mass %to less than 9.0 mass %, the coating solution hardly permeated thesubstrate, whereby adhesin between the substrate and the coatingsolution lowered, cutting was difficult, and fluff remained on the edgesurfaces. Further, it was found that a high linear pressure had to beapplied at the time of controlling the thickness of the substrate to apredetermined value by heat calender, thereby making it difficult toreduce the thickness of the substrate. As a result, internal resistancedeteriorated. Since the number of fine fibril fibers increased too much,bonding between fibers became loose, thereby deteriorating strengthcharacteristic.

In Comparative Example 4-8, 5.0 mass % of the fibrillated heat-resistantfibers (II) and 3.9 mass % of the fibrid were contained. Although thetotal content of the heat-resistant fibers was not less than 2.0 mass %to less than 9.0 mass %, the coating solution hardly permeated thesubstrate, whereby adhesin between the substrate and the coatingsolution lowered, cutting was difficult, and fluff remained on the edgesurfaces. Further, it was found that a high linear pressure had to beapplied at the time of controlling the thickness of the substrate to apredetermined value by heat calender, thereby making it difficult toreduce the thickness of the substrate. As a result, internal resistancedeteriorated. Since the number of thick stem fibers increased, bondingbetween fibers became loose, thereby deteriorating strengthcharacteristic.

In «Example 5» and «Example 6», the following physical properties ofsubstrates for lithium ion battery separators and lithium ion batteryseparators were measured and evaluated.

<Basis Weight of Substrate and Coating Weight of Inorganic ParticleLayer>

The basis weights of the substrates and the separators were measured inaccordance with JIS P8124. The coating weight of the inorganic particlelayer was calculated by subtracting the basis weight of the substratefrom the basis weight of the separator.

<Thicknesses of Substrate and Separator>

The thicknesses of the substrate and the separator were measured under aload of 5 N by using an external micrometer specified in JIS B7502.

[Evaluation of Pinhole]

Each A4-sized separator was checked visually for the state of a pinholeby using transmitted light and evaluated based on the followingcriteria.

◯: no pinhole is seenΔ: transmitted light is partially seen slightlyX: many transmitted light is seen obviously

[Evaluation of Strength of Inorganic Particle Layer]

A test piece measuring 100 mm in a width direction×200 m in a flowdirection was cut out from each separator, folded at a position 100 mmin the flow direction and bent at 180°. Thereafter, it was unfolded toreturn to the original state three times, and a crack in the inorganicparticle layer was checked visually and evaluated based on the followingcriteria. In «Example 5», the test piece was bent in such a manner thatthe substrate surface was located on the inner side and the inorganicparticle layer surface was located on the outer side. In «Example 6»,the test piece was bent in such a manner that the inorganic particlelayer A was located on the inner side and the inorganic particle layer Bwas located on the outer side.

◯: damage to fold of inorganic particle layer is not seenX: a large number of cracks are seen in fold of inorganic particle layer

[Cuttability of Separator]

Each separator was slit into 8 rolls measuring 50 mm in a widthdirection×200 m in a flow direction to check the both edge surfacesvisually and evaluated based on the following criteria.

◯: no fluff is seen on both edge surfacesX: fluff is seen on both edge surfaces

[Heat Resistance of Separator]

A test piece measuring 100 mm in a width direction×100 mm in a flowdirection was cut out from each separator and left to stand in a 200° C.constant-temperature drier for 1 hour to measure the length of a centralpart in the flow direction. The length was compared with the lengthbefore the test to obtain the percentage of a reduction in length asthermal shrinkage and evaluate heat resistance based on the followingcriteria.

◯: thermal shrinkage is less than 1.0%X: thermal shrinkage is not less than 1.0%

[Manufacture of Battery for Evaluation]

Each separator was used to manufacture a battery for evaluation having adesign capacity of 30 mAh by using a three-element cathode material(LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂) as a cathode, graphite as an anode and a1 mol/L ethylene carbonate (EC)/diethyl carbonate (DEC)/dimethylcarbonate (DMC) (volume ratio of 1/1/1) mixed solvent solution oflithium hexafluorophosphate (LiPF₆) as an electrolytic solution. In«Example 5», the inorganic particle layer surface was opposed to theanode. In «Example 6», the inorganic particle layer B was opposed to theanode.

[Evaluation of Internal Resistance]

After three cycles of shake-down charging/discharging, each consistingof “30 mA constant current charging, 4.2 V constant voltage charging (1hour), 30 mA constant current discharging and next cycle when thevoltage became 2.7 V”, were carried out for each battery, “30 mAconstant current charging, 4.2 V constant voltage charging (1 hour), 30minutes of 6 mA constant current discharging (discharge amount of 3mAh), the measurement of voltage (voltage “a”) right before the end ofdischarging, 30 mA constant current charging, 4.2 V constant voltagecharging (1 hour), 72 seconds of 150 mA constant current discharging(discharge amount of 3 mAh), and the measurement of voltage (voltage“b”) right before the end of discharging” were carried out to obtaininternal resistance from the expression “internal resistance Ω=(voltage“a”−voltage “b”)/(150 mA−6 mA)”.

◯: internal resistance of less than 2.8ΩΔ: internal resistance of not less than 2.8Ω to less than 3.5ΩX: internal resistance of not less than 3.5Ω

Example 5

<Preparation of Coating Solution a1>

100 parts of magnesium hydroxide having an average particle diameter of2.0 μm was dispersed in 150 parts by mass of water. Then, 75 parts bymass of a 2 mass % aqueous solution of a carboxymethyl cellulose sodiumsalt having a viscosity at 25° C. of a 1 mass % aqueous solution thereofof 200 mPa·s was added to and mixed with the dispersion under agitation.Subsequently, 10 parts by mass of a carboxy modified styrene-butadienecopolymer resin emulsion having a glass transition point of −18° C. andan average particle diameter of 0.2 μm (solid concentration of 50 mass%) was added to and mixed with the resulting mixture under agitation.Finally, conditioned water was added to a solid concentration of 25 mass% to prepare a coating solution a1.

<Preparation of Coating Solution a2>

A coating solution a2 was prepared in the same manner as the preparationof the coating solution a1 except that magnesium hydroxide having anaverage particle diameter of 3.0 μm was used in place of magnesiumhydroxide having an average particle diameter of 2.0 μm.

<Preparation of Coating Solution a3>

A coating solution a3 was prepared in the same manner as the preparationof the coating solution a1 except that magnesium hydroxide having anaverage particle diameter of 4.0 μm was used in place of magnesiumhydroxide having an average particle diameter of 2.0 μm.

<Preparation of Coating Solution b1>

100 parts of magnesium hydroxide having an average particle diameter of0.5 μm was dispersed in 150 parts by mass of water. Then, 75 parts bymass of a 2 mass % aqueous solution of a carboxymethyl cellulose sodiumsalt having a viscosity at 25° C. of a 1 mass % aqueous solution thereofof 200 mPa·s was added to and mixed with the dispersion under agitation.Subsequently, 10 parts by mass of a carboxy modified styrene-butadienecopolymer resin emulsion having a glass transition point of −18° C. andan average particle diameter of 0.2 μm (solid concentration of 50 mass%) was added to and mixed with the resulting mixture under agitation.Finally, conditioned water was added to a solid concentration of 25 mass% to prepare a coating solution b1.

<Preparation of Coating Solution b2>

A coating solution b2 was prepared in the same manner as the preparationof the coating solution b1 except that magnesium hydroxide having anaverage particle diameter of 1.0 μm was used in place of magnesiumhydroxide having an average particle diameter of 0.5 μm.

<Preparation of Coating Solution b3>

A coating solution b3 was prepared in the same manner as the preparationof the coating solution b1 except that magnesium hydroxide having anaverage particle diameter of 1.5 μm was used in place of magnesiumhydroxide having an average particle diameter of 0.5 μm.

<Lithium Ion Battery Separator>

Example 5-1

After the coating solution a2 was applied to the substrate 1-1 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the same coating surface to a coating weight of 10.0 g/m²with a kiss reverse gravure coater and dried to form an inorganicparticle layer B, thereby obtaining the separator of Example 5-1.

Example 5-2

After the coating solution a2 was applied to the substrate 1-2 to acoating weight of 6.5 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the same coating surface to a coating weight of 7.5 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Example 5-2.

Example 5-3

After the coating solution a1 was applied to the substrate 1-4 to acoating weight of 3.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b1 wasapplied to the same coating surface to a coating weight of 3.5 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Example 5-3.

Example 5-4

After the coating solution a2 was applied to the substrate 1-3 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the same coating surface to a coating weight of 6.0 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Example 5-4.

Example 5-5

After the coating solution a1 was applied to the substrate 1-14 to acoating weight of 2.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b1 wasapplied to the same coating surface to a coating weight of 2.5 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Example 5-5.

Example 5-6

After the coating solution a3 was applied to the substrate 1-15 to acoating weight of 10.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b3 wasapplied to the same coating surface to a coating weight of 10.0 g/m²with a kiss reverse gravure coater and dried to form an inorganicparticle layer B, thereby obtaining the separator of Example 5-6.

Example 5-7

After the coating solution a2 was applied to the substrate 1-11 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the same coating surface to a coating weight of 6.0 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Example 5-7.

Example 5-8

After the coating solution a2 was applied to the substrate 1-1 to acoating weight of 10.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the same coating surface to a coating weight of 5.0 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Example 5-8.

Example 5-9

After the coating solution a2 was applied to the substrate 1-1 to acoating weight of 7.5 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the same coating surface to a coating weight of 7.5 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Example 5-9.

Comparative Example 5-1

After the coating solution a2 was applied to the substrate 1-5 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the same coating surface to a coating weight of 6.0 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Comparative Example 5-1.

Comparative Example 5-2

After the coating solution a2 was applied to the substrate 1-6 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the same coating surface to a coating weight of 6.0 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Comparative Example 5-2.

Comparative Example 5-3

After the coating solution a2 was applied to the substrate 1-7 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the same coating surface to a coating weight of 6.0 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Comparative Example 5-3.

Comparative Example 5-4

After the coating solution a2 was applied to the substrate 1-8 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the same coating surface to a coating weight of 6.0 g/m² witha kiss reverse gravure coater and dried to form an inorganic particlelayer B, thereby obtaining the separator of Comparative Example 5-4.

TABLE 5 Ex. 5-1 Ex. 5-2 Ex. 5-3 Ex. 5-4 Ex. 5-5 Sub. 1-1 Sub. 1-2 Sub.1-4 Sub. 1-3 Sub. 1-14 Fiber blending oriented crystallized pbm 55.559.0 55.5 55.5 57.0 PET-based synthetic resin short fibers 0.06 dtex × 3mm Undrawn PET-based synthetic pbm 40.0 40.0 40.0 40.0 40.0 resin shortfibers 0.2 dtex × 3 mm Fibrillated heat-resistant pbm 4.5 1.0 4.5 4.53.0 fibers Beaten melt spun cellulose pbm 0.0 0.0 0.0 0.0 0.0 fibersFibrillated Modified freeness ml 50 50 250 50 50 heat-resistant fibersMass-weighted average fiber mm 0.58 0.58 0.82 0.58 0.58 lengthLength-weighted average mm 0.33 0.33 0.45 0.33 0.33 fiber length Fiberwidth μm 15.5 15.5 19.5 15.5 15.5 Linear pressure of heat calender kN/m100 100 100 100 100 Physical Basis weight of substrate g/m² 10 10 8 8 7properties Thickness of substrate μm 15 15 11 11 10 Coating weight ofinorganic g/m² 5.0 6.5 3.0 5.0 2.0 particle layer A Coating weight ofinorganic g/m² 10.0 7.5 3.5 6.0 2.5 particle layer B Thickness ofseparator μm 26 26 16 20 13 Evaluation pinhole — ◯ ◯ ◯ ◯ Δ strength ofinorganic particle — ◯ ◯ ◯ ◯ ◯ layer Cuttability of separator — ◯ ◯ ◯ ◯◯ heat resistance — ◯ ◯ ◯ ◯ ◯ Internal resistance — ◯ ◯ ◯ ◯ ◯ Ex. 5-6Ex. 5-7 Ex. 5-8 Ex. 5-9 Sub. 1-15 Sub. 1-11 Sub. 1-1 Sub. 1-1 Fiberblending oriented crystallized pbm 56.0 50.0 55.5 55.5 PET-basedsynthetic resin short fibers 0.06 dtex × 3 mm Undrawn PET-basedsynthetic pbm 40.0 40.0 40.0 40.0 resin short fibers 0.2 dtex × 3 mmFibrillated heat-resistant pbm 4.0 4.5 4.5 4.5 fibers Beaten melt spuncellulose pbm 0.0 5.5 0.0 0.0 fibers Fibrillated Modified freeness ml 5050 50 50 heat-resistant Mass-weighted average fiber mm 0.58 0.58 0.580.58 fibers length Length-weighted average mm 0.33 0.33 0.33 0.33 fiberlength Fiber width μm 15.5 15.5 15.5 15.5 Linear pressure of heatcalender kN/m 100 100 100 100 Physical properties Basis weight ofsubstrate g/m² 12 8 10 10 Thickness of substrate μm 18 11 15 15 Coatingweight of inorganic g/m² 10.0 5.0 10.0 7.5 particle layer A Coatingweight of inorganic g/m² 10.0 6.0 5.0 7.5 particle layer B Thickness ofseparator μm 35 20 28 27 Evaluation pinhole — ◯ ◯ ◯ ◯ strength ofinorganic particle — ◯ ◯ ◯ ◯ layer Cuttability of separator — ◯ ◯ ◯ ◯heat resistance — ◯ ◯ ◯ ◯ Internal resistance — Δ ◯ ◯ ◯ C. Ex. 5-1 C.Ex. 5-2 C. Ex. 5-3 C. Ex. 5-4 Sub. 1-5 Sub. 1-6 Sub. 1-7 Sub. 1-8 Fiberblending oriented crystallized pbm 60.0 55.0 50.0 55.5 PET-basedsynthetic resin short fibers 0.06 dtex × 3 mm Undrawn PET-basedsynthetic pbm 40.0 40.0 40.0 40.0 resin short fibers 0.2 dtex × 3 mmFibrillated heat-resistant pbm 0.0 5.0 10.0 4.5 fibers Beaten melt spuncellulose pbm 0.0 0.0 0.0 0.0 fibers Fibrillated Modified freeness ml —50 50 320 heat-resistant Mass-weighted average fiber mm — 0.58 0.58 1.30fibers length Length-weighted average mm — 0.33 0.33 0.59 fiber lengthFiber width μm — 15.5 15.5 25.2 Linear pressure of heat calender kN/m100 120 175 100 Physical properties Basis weight of substrate g/m² 8 8 88 Thickness of substrate μm 11 11 11 11 Coating weight of inorganic g/m²5.0 5.0 5.0 5.0 particle layer A Coating weight of inorganic g/m² 6.06.0 6.0 6.0 particle layer B Thickness of separator μm 20 20 20 20Evaluation pinhole — ◯ ◯ ◯ ◯ strength of inorganic particle — ◯ ◯ ◯ ◯layer Cuttability of separator — ◯ X X X heat resistance — X ◯ ◯ ◯Internal resistance — ◯ X X ◯ Ex.: Example Sub.: Substrate pbm: part bymass

Since the separators manufactured in Examples 5-1 to 5-9 contained thefibrillated heat-resistant fibers (I) and the synthetic resin shortfibers and had such constitution that the inorganic particle layer Acontaining magnesium hydroxide having an average particle diameter of2.0 to 4.0 μm and the inorganic particle layer B containing magnesiumhydroxide having an average particle diameter of not less than 0.5 μm toless than 2.0 μm were formed on one side of the substrate having acontent of the fibrillated heat-resistant fibers (I) of not less than1.0 mass % to less than 5.0 mass % based on the total of all the fibercomponent contained in the substrate in the mentioned order, they wereexcellent in pinhole, the strength of the inorganic particle layers,cuttability, heat resistance and internal resistance.

Meanwhile, since the separator manufactured in Comparative Example 5-1did not contain the fibrillated heat-resistant fibers (I), it wasinferior in heat resistance.

Since the separators manufactured in Comparative Examples 5-2 and 5-3had a content of the fibrillated heat-resistant fibers (I) of not lessthan 5.0 mass %, they were inferior in cuttability and internalresistance.

Since the separator manufactured in Comparative Example 5-4 did notcontain the fibrillated heat-resistant fibers (I) but contained only thefibrillated heat-resistant fibers (II), it was inferior in cuttability.

Since the separator manufactured in Example 5-5 had rather low coatingweights of the inorganic particle layer A and the inorganic particlelayer B, as compared with the separators of Examples 5-1 to 5-4 andExamples 5-6 to 5-9, transmitted light was partially seen slightly inthe evaluation of a pinhole but there was no problem with actual use.

Since the separator manufactured in Example 5-6 had rather high coatingweights of the inorganic particle layer A and the inorganic particle B,as compared with the separators of Examples 5-1 to 5-5 and Examples 5-7to 5-9, internal resistance was slightly high but there was no problemwith actual use.

The separators manufactured in Example 5-1, Example 5-8 and Example 5-9had the same total coating weight of the inorganic particle layer A andthe inorganic particle layer B but differed in the ratio of the coatingweight of the inorganic particle layer A to the coating weight of theinorganic particle layer B. Since the average particle diameter ofmagnesium hydroxide contained in the inorganic particle layer A islarger than the average particle diameter of magnesium hydroxidecontained in the inorganic particle layer B, the inorganic particlelayer A becomes thicker than the inorganic particle layer B in the caseof the same coating weight. Therefore, as for the thickness of theseparator, the separator of Example 5-1 in which the coating weight ofthe inorganic particle layer A was lower than the coating weight of theinorganic particle layer B was thinner than the separator of Example 5-9in which the coating weight of the inorganic particle layer A and thecoating weight of the inorganic particle layer B were the same, and theseparator of the Example 5-9 was thinner than the separator of Example5-8 in which the coating weight of the inorganic particle layer A washigher than the coating weight of the inorganic particle layer B.

Example 6

<Lithium Ion Battery Separator>

Example 6-1

After the coating solution a2 was applied to the substrate 1-1 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the other side of the substrate 1-1 to a coating weight of10.0 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator of Example6-1.

Example 6-2

After the coating solution a2 was applied to the substrate 1-2 to acoating weight of 6.5 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the other side of the substrate 1-2 to a coating weight of7.5 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator of Example6-2.

Example 6-3

After the coating solution a1 was applied to the substrate 1-4 to acoating weight of 3.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b1 wasapplied to the other side of the substrate 1-4 to a coating weight of3.5 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator of Example6-3.

Example 6-4

After the coating solution a2 was applied to the substrate 1-3 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the other side of the substrate 1-3 to a coating weight of6.0 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator of Example6-4.

Example 6-5

After the coating solution a1 was applied to the substrate 1-14 to acoating weight of 2.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b1 wasapplied to the other side of the substrate 1-14 to a coating weight of2.5 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator of Example6-5.

Example 6-6

After the coating solution a3 was applied to the substrate 1-15 to acoating weight of 10.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b3 wasapplied to the other side of the substrate 1-15 to a coating weight of10.0 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator of Example6-6.

Example 6-7

After the coating solution a2 was applied to the substrate 1-11 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the other side of the substrate 1-11 to a coating weight of6.0 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator of Example6-7.

Example 6-8

After the coating solution a2 was applied to the substrate 1-1 to acoating weight of 10.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the other side of the substrate 1-1 to a coating weight of5.0 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator of Example6-8.

Example 6-9

After the coating solution a2 was applied to the substrate 1-1 to acoating weight of 7.5 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the other side of the substrate 1-1 to a coating weight of7.5 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator of Example6-9.

Comparative Example 6-1

After the coating solution a2 was applied to the substrate 1-5 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the other side of the substrate 1-5 to a coating weight of6.0 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator ofComparative Example 6-1.

Comparative Example 6-2

After the coating solution a2 was applied to the substrate 1-6 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the other side of the substrate 1-6 to a coating weight of6.0 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator ofComparative Example 6-2.

Comparative Example 6-3

After the coating solution a2 was applied to the substrate 1-7 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the other side of the substrate 1-7 to a coating weight of6.0 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator ofComparative Example 6-3.

Comparative Example 6-4

After the coating solution a2 was applied to the substrate 1-8 to acoating weight of 5.0 g/m² with a kiss reverse gravure coater and driedto form an inorganic particle layer A, the coating solution b2 wasapplied to the other side of the substrate 1-8 to a coating weight of6.0 g/m² with a kiss reverse gravure coater and dried to form aninorganic particle layer B, thereby obtaining the separator ofComparative Example 6-4.

TABLE 6 Ex. 6-1 Ex. 6-2 Ex. 6-3 Ex. 6-4 Ex. 6-5 Sub. 1-1 Sub. 1-2 Sub.1-4 Sub. 1-3 Sub. 1-14 Fiber blending oriented crystallized pbm 55.559.0 55.5 55.5 57.0 PET-based synthetic resin short fibers 0.06 dtex × 3mm Undrawn PET-based synthetic pbm 40.0 40.0 40.0 40.0 40.0 resin shortfibers 0.2 dtex × 3 mm Fibrillated heat-resistant pbm 4.5 1.0 4.5 4.53.0 fibers Beaten melt spun cellulose pbm 0.0 0.0 0.0 0.0 0.0 fibersFibrillated Modified freeness ml 50 50 250 50 50 heat-resistantMass-weighted average fiber mm 0.58 0.58 0.82 0.58 0.58 fibers lengthLength-weighted average mm 0.33 0.33 0.45 0.33 0.33 fiber length Fiberwidth μm 15.5 15.5 19.5 15.5 15.5 Linear pressure of heat calender kN/m100 100 100 100 100 Physical Basis weight of substrate g/m² 10 10 8 8 7properties Thickness of substrate μm 15 15 11 11 10 Coating weight ofinorganic g/m² 5.0 6.5 3.0 5.0 2.0 particle layer A Coating weight ofinorganic g/m² 10.0 7.5 3.5 6.0 2.5 particle layer B Thickness ofseparator μm 25 25 15 19 13 Evaluation pinhole — ◯ ◯ ◯ ◯ Δ strength ofinorganic particle — ◯ ◯ ◯ ◯ ◯ layer Cuttability of separator — ◯ ◯ ◯ ◯◯ heat resistance — ◯ ◯ ◯ ◯ ◯ Internal resistance — ◯ ◯ ◯ ◯ ◯ Ex. 6-6Ex. 6-7 Ex. 6-8 Ex. 6-9 Sub. 1-15 Sub. 1-11 Sub. 1-1 Sub. 1-1 Fiberblending oriented crystallized pbm 56.0 50.0 55.5 55.5 PET-basedsynthetic resin short fibers 0.06 dtex × 3 mm Undrawn PET-basedsynthetic pbm 40.0 40.0 40.0 40.0 resin short fibers 0.2 dtex × 3 mmFibrillated heat-resistant pbm 4.0 4.5 4.5 4.5 fibers Beaten melt spuncellulose pbm 0.0 5.5 0.0 0.0 fibers Fibrillated Modified freeness ml 5050 50 50 heat-resistant Mass-weighted average fiber mm 0.58 0.58 0.580.58 fibers length Length-weighted average mm 0.33 0.33 0.33 0.33 fiberlength Fiber width μm 15.5 15.5 15.5 15.5 Linear pressure of heatcalender kN/m 100 100 100 100 Physical properties Basis weight ofsubstrate g/m² 12 8 10 10 Thickness of substrate μm 18 11 15 15 Coatingweight of inorganic g/m² 10.0 5.0 10.0 7.5 particle layer A Coatingweight of inorganic g/m² 10.0 6.0 5.0 7.5 particle layer B Thickness ofseparator μm 35 19 27 26 Evaluation pinhole — ◯ ◯ ◯ ◯ strength ofinorganic particle — ◯ ◯ ◯ ◯ layer Cuttability of separator — ◯ ◯ ◯ ◯heat resistance — ◯ ◯ ◯ ◯ Internal resistance — Δ ◯ ◯ ◯ C. Ex. 6-1 C.Ex. 6-2 C. Ex. 6-3 C. Ex. 6-4 Sub. 1-5 Sub. 1-6 Sub. 1-7 Sub. 1-8 Fiberblending oriented crystallized pbm 60.0 55.0 50.0 55.5 PET-basedsynthetic resin short fibers 0.06 dtex × 3 mm Undrawn PET-basedsynthetic pbm 40.0 40.0 40.0 40.0 resin short fibers 0.2 dtex × 3 mmFibrillated heat-resistant Pbm 0.0 5.0 10.0 4.5 fibers Beaten melt spuncellulose pbm 0.0 0.0 0.0 0.0 fibers Fibrillated Modified freeness ml —50 50 320 heat-resistant Mass-weighted average fiber mm — 0.58 0.58 1.30fibers length Length-weighted average mm — 0.33 0.33 0.59 fiber lengthFiber width μm — 15.5 15.5 25.2 Linear pressure of heat calender kN/m100 120 175 100 Physical properties Basis weight of substrate g/m² 8 8 88 Thickness of substrate μm 11 11 11 11 Coating weight of inorganic g/m²5.0 5.0 5.0 5.0 particle layer A Coating weight of inorganic g/m² 6.06.0 6.0 6.0 particle layer B Thickness of separator μm 19 19 19 19Evaluation pinhole — ◯ ◯ ◯ ◯ strength of inorganic particle — ◯ ◯ ◯ ◯layer Cuttability of separator — ◯ X X X heat resistance — X ◯ ◯ ◯Internal resistance — ◯ X X ◯ Ex.: Example Sub.: Substrate pbm: part bymass

The separators manufactured in Examples 6-1 to 6-9 had the substratewhich contained the fibrillated heat-resistant fibers (I) and thesynthetic resin short fibers and had a content of the fibrillatedheat-resistant fibers (I) of not less than 1.0 mass % to less than 5mass % based on the total of all the fiber components contained in thesubstrate, the inorganic particle layer A containing magnesium hydroxidehaving an average particle diameter of 2.0 to 4.0 μm on one side of thesubstrate and the inorganic particle layer B containing magnesiumhydroxide having an average particle diameter of not less than 0.5 μm toless than 2.0 μm on the other side. Therefore, they were excellent inpinhole, the strength of inorganic particle layers, cuttability, heatresistance and internal resistance.

Meanwhile, since the separator manufactured in Comparative Example 6-1did not contain the fibrillated heat-resistant fibers (I), it wasinferior in heat resistance.

Since the separators manufactured in Comparative Examples 6-2 and 6-3had a content of the fibrillated heat-resistant fibers (I) of not lessthan 5.0 mass %, they were inferior in cuttability and internalresistance.

Since the separator manufactured in Comparative Example 6-4 did notcontain the fibrillated heat-resistant fibers (I) but contained only thefibrillated heat-resistant fibers (II), it was inferior in cuttability.

Since the separator manufactured in Example 6-5 had rather low coatingweights of the inorganic particle A and the inorganic particle B, ascompared with the separators of Examples 6-1 to 6-4 and Examples 6-6 to6-9, transmitted light was partially seen slightly in the evaluation ofa pinhole but there was no problem with actual use.

Since the separator manufactured in Example 6-6 had rather high coatingweights of the inorganic particle layer A and the inorganic particlelayer B, as compared with the separators of Examples 6-1 to 6-5 andExamples 6-7 to 6-9, internal resistance was a little higher but therewas no problem with actual use.

The separators manufactured in Example 6-1, Example 6-8 and Example 6-9had the same total coating weight of the inorganic particle layer A andthe inorganic particle layer B but differed in the ratio of the coatingweight of the inorganic particle layer A to the coating weight of theinorganic particle layer B. Since the average particle diameter ofmagnesium hydroxide contained in the inorganic particle layer A islarger than the average particle diameter of magnesium hydroxidecontained in the inorganic particle layer B, in the case of the samecoating weight, the inorganic particle layer A becomes thicker than theinorganic particle layer B. As for the thickness of the separator, theseparator of Example 6-1 in which the coating weight of the inorganicparticle layer A was lower than the coating weight of the inorganicparticle layer B was thinner than the separator of Example 6-9 in whichthe coating weight of the inorganic particle layer A and the coatingweight of the inorganic particle layer B were the same, and theseparator of Example 6-9 was thinner than the separator of Example 6-8in which the coating weight of the inorganic particle layer A was higherthan the coating weight of the inorganic particle layer B.

INDUSTRIAL FEASIBILITY

The substrate for lithium ion battery separators of the presentinvention and the lithium ion battery separator of the present inventioncan be advantageously used in lithium ion secondary batteries such aslithium ion secondary batteries and lithium ion polymer secondarybatteries.

EXPLANATION OF REFERENCE NUMERALS

-   -   1 fibrillated heat-resistant fibers having a modified freeness        of not more than 300 ml    -   2 fibrillated heat-resistant fibers having a modified freeness        of more than 300 ml    -   3 fibrid    -   4 synthetic resin short fibers

1. A substrate for lithium ion battery separators, comprising: heat-resistant fibers, and synthetic resin short fibers, wherein the heat-resistant fibers comprise first fibrillated heat-resistant fibers, having a modified freeness of not more than 300 ml and second fibrillated heat-resistant fibers, having a modified freeness of more than 300 ml, wherein the content of the first fibrillated heat-resistant fibers is not less than 1.0 mass % to less than 5.0 mass % based on a total of all fiber components contained in the substrate, wherein the modified freeness is a value measured in accordance with JIS P8121-2:2012 except that an 80-mesh wire net having a wire diameter of 0.14 mm and an opening of 0.18 mm is used as a screening plate and the concentration of a sample is 0.1%, and wherein the synthetic resin short fibers have a fiber length of 1 to 10 mm.
 2. The substrate for lithium ion battery separators according to claim 1, wherein the total content of the first fibrillated heat-resistant fibers and the second fibrillated heat-resistance fibers is not less than 2.0 mass % to less than 9.0 mass % based on the total of all fiber components contained in the substrate.
 3. The substrate for lithium ion battery separators according to claim 2, wherein the content of the second fibrillated heat-resistant fibers is not less than 1.0 mass % to less than 5.0 mass % based on the total of all fiber components contained in the substrate.
 4. The substrate for lithium ion battery separators according to claim 3, wherein the content of the synthetic resin short fibers is 90.0 to 98.0 mass % based on the total of all fiber components contained in the substrate.
 5. A substrate for lithium ion battery separators, comprising: heat-resistant fibers, and synthetic resin short fibers, wherein the heat-resistant fibers comprise fibrillated heat-resistant fibers, having a modified freeness of not more than 300 ml, and fibrid made of para-aromatic polyamide, having a modified freeness of not more than 300 ml, wherein the content of the heat-resistant fibers is not less than 1.0 mass % to less than 5.0 mass % based on a total of all fiber components contained in the substrate, wherein the modified freeness is a value measured in accordance with JIS P8121-2:2012 except that an 80-mesh wire net having a wire diameter of 0.14 mm and an opening of 0.18 mm is used as a screening plate and the concentration of a sample is 0.1%, and wherein the synthetic resin short fibers have a fiber length of 1 to 10 mm.
 6. The substrate for lithium ion battery separators according to claim 5, wherein the content of the synthetic resin short fibers is 90.0 to 99.0 mass % based on the total of all fiber components contained in the substrate.
 7. The substrate for lithium ion battery separators according to claim 5, wherein the content of the synthetic resin short fibers is more than 95.0 mass % to not more than 99.0 mass % based on the total of all fiber components contained in the substrate.
 8. A substrate for lithium ion battery separators, comprising: heat-resistant fibers, and synthetic resin short fibers, wherein the heat-resistant fibers comprise first fibrillated heat-resistant fibers, having a modified freeness of not more than 300 ml, fibrid made of para-aromatic polyamide, having a modified freeness of not more than 300 ml, and second fibrillated heat-resistant fibers, having a modified freeness of more than 300 ml, and wherein the content of the heat-resistant fibers is not less than 2.0 mass % to less than 9.0 mass % based on a total of all fiber components contained in the substrate, wherein the modified freeness is a value measured in accordance with JIS P8121-2:2012 except that an 80-mesh wire net having a wire diameter of 0.14 mm and an opening of 0.18 mm is used as a screening plate and the concentration of a sample is 0.1% and wherein the synthetic resin short fibers have a fiber length of 1 to 10 mm.
 9. The substrate for lithium ion battery separators according to claim 8, wherein the content of the first fibrillated heat-resistant fibers is not less than 0.6 mass % to less than 4.6 mass %, the content of the fibrid is not less than 0.4 mass % to less than 2.5 mass %, and the content of the second fibrillated heat-resistant fibers is not less than 1.0 mass % to less than 4.0 mass % based on the total of all fiber components contained in the substrate.
 10. The substrate for lithium ion battery separators according to claim 8, wherein the content of the synthetic resin short fibers is 90.0 to 98.0 mass % based on the total of all fiber components contained in the substrate.
 11. A lithium ion battery separator which is a combination of at least one composite selected from the group consisting of an inorganic particle layer containing inorganic particles, organic particle layer containing organic particles, porous film, microfiber layer, solid electrolyte and gel-like electrolyte and the substrate for lithium ion battery separators according to claim
 1. 12. A lithium ion battery separator which is a combination of at least one composite selected from the group consisting of an inorganic particle layer containing inorganic particles, organic particle layer containing organic particles, porous film, microfiber layer, solid electrolyte and gel-like electrolyte and the substrate for lithium ion battery separators according to claim
 5. 13. A lithium ion battery separator which is a combination of at least one composite selected from the group consisting of an inorganic particle layer containing inorganic particles, organic particle layer containing organic particles, porous film, microfiber layer, solid electrolyte and gel-like electrolyte and the substrate for lithium ion battery separators according to claim
 8. 